diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index b7c0eb71..72de85fc 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -171,6 +171,9 @@ jobs:
           - name: Check package manifest
             tox: check-manifest
             run-if: true
+          - name: Check mypy
+            tox: typecheck
+            run-if: true
 
     steps:
     - uses: actions/checkout@v3
diff --git a/pyproject.toml b/pyproject.toml
index b7a31f45..b15dbf68 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -57,6 +57,19 @@ exclude = '''
 
 [tool.isort]
 profile = "attrs"
+line_length = 88
+
+
+[tool.mypy]
+strict = true
+# 2022-09-04: Trial's API isn't annotated yet, which limits the usefulness of type-checking
+#             the unit tests. Therefore they have not been annotated yet.
+exclude = '^src/towncrier/test/.*\.py$'
+
+[[tool.mypy.overrides]]
+module = 'click_default_group'
+# 2022-09-04: This library has no type annotations.
+ignore_missing_imports = true
 
 
 [build-system]
diff --git a/src/towncrier/__init__.py b/src/towncrier/__init__.py
index ba09c39a..ab8ced7c 100644
--- a/src/towncrier/__init__.py
+++ b/src/towncrier/__init__.py
@@ -5,6 +5,8 @@
 towncrier, a builder for your news files.
 """
 
+from __future__ import annotations
+
 from ._version import __version__
 
 
diff --git a/src/towncrier/__main__.py b/src/towncrier/__main__.py
index e7257122..cf8d9378 100644
--- a/src/towncrier/__main__.py
+++ b/src/towncrier/__main__.py
@@ -1,3 +1,5 @@
+from __future__ import annotations
+
 from towncrier._shell import cli
 
 
diff --git a/src/towncrier/_builder.py b/src/towncrier/_builder.py
index 42c3bb00..af4663ad 100644
--- a/src/towncrier/_builder.py
+++ b/src/towncrier/_builder.py
@@ -2,18 +2,21 @@
 # See LICENSE for details.
 
 
+from __future__ import annotations
+
 import os
 import textwrap
 import traceback
 
 from collections import OrderedDict
+from typing import Any, Iterator, Mapping, Sequence
 
 from jinja2 import Template
 
 from ._settings import ConfigError
 
 
-def strip_if_integer_string(s):
+def strip_if_integer_string(s: str) -> str:
     try:
         i = int(s)
     except ValueError:
@@ -24,7 +27,9 @@ def strip_if_integer_string(s):
 
 # Returns ticket, category and counter or (None, None, None) if the basename
 # could not be parsed or doesn't contain a valid category.
-def parse_newfragment_basename(basename, definitions):
+def parse_newfragment_basename(
+    basename: str, definitions: Sequence[str]
+) -> tuple[str, str, int] | tuple[None, None, None]:
     invalid = (None, None, None)
     parts = basename.split(".")
 
@@ -74,7 +79,12 @@ def parse_newfragment_basename(basename, definitions):
 # We should really use attrs.
 #
 # Also returns a list of the paths that the fragments were taken from.
-def find_fragments(base_directory, sections, fragment_directory, definitions):
+def find_fragments(
+    base_directory: str,
+    sections: Mapping[str, str],
+    fragment_directory: str | None,
+    definitions: Sequence[str],
+) -> tuple[Mapping[str, Mapping[tuple[str, str, int], str]], list[str]]:
     """
     Sections are a dictonary of section names to paths.
     """
@@ -105,6 +115,8 @@ def find_fragments(base_directory, sections, fragment_directory, definitions):
             )
             if category is None:
                 continue
+            assert ticket is not None
+            assert counter is not None
 
             full_filename = os.path.join(section_dir, basename)
             fragment_filenames.append(full_filename)
@@ -124,12 +136,12 @@ def find_fragments(base_directory, sections, fragment_directory, definitions):
     return content, fragment_filenames
 
 
-def indent(text, prefix):
+def indent(text: str, prefix: str) -> str:
     """
     Adds `prefix` to the beginning of non-empty lines in `text`.
     """
     # Based on Python 3's textwrap.indent
-    def prefixed_lines():
+    def prefixed_lines() -> Iterator[str]:
         for line in text.splitlines(True):
             yield (prefix + line if line.strip() else line)
 
@@ -139,12 +151,16 @@ def prefixed_lines():
 # Takes the output from find_fragments above. Probably it would be useful to
 # add an example output here. Next time someone digs deep enough to figure it
 # out, please do so...
-def split_fragments(fragments, definitions, all_bullets=True):
+def split_fragments(
+    fragments: Mapping[str, Mapping[tuple[str, str, int], str]],
+    definitions: Mapping[str, Mapping[str, Any]],
+    all_bullets: bool = True,
+) -> Mapping[str, Mapping[str, Mapping[str, Sequence[str]]]]:
 
     output = OrderedDict()
 
     for section_name, section_fragments in fragments.items():
-        section = {}
+        section: dict[str, dict[str, list[str]]] = {}
 
         for (ticket, category, counter), content in section_fragments.items():
 
@@ -174,7 +190,7 @@ def split_fragments(fragments, definitions, all_bullets=True):
     return output
 
 
-def issue_key(issue):
+def issue_key(issue: str) -> tuple[int, str]:
     # We want integer issues to sort as integers, and we also want string
     # issues to sort as strings. We arbitrarily put string issues before
     # integer issues (hopefully no-one uses both at once).
@@ -185,12 +201,12 @@ def issue_key(issue):
         return (-1, issue)
 
 
-def entry_key(entry):
+def entry_key(entry: tuple[str, Sequence[str]]) -> list[tuple[int, str]]:
     _, issues = entry
     return [issue_key(issue) for issue in issues]
 
 
-def bullet_key(entry):
+def bullet_key(entry: tuple[str, Sequence[str]]) -> int:
     text, _ = entry
     if not text:
         return -1
@@ -203,7 +219,7 @@ def bullet_key(entry):
     return 3
 
 
-def render_issue(issue_format, issue):
+def render_issue(issue_format: str | None, issue: str) -> str:
     if issue_format is None:
         try:
             int(issue)
@@ -215,24 +231,24 @@ def render_issue(issue_format, issue):
 
 
 def render_fragments(
-    template,
-    issue_format,
-    fragments,
-    definitions,
-    underlines,
-    wrap,
-    versiondata,
-    top_underline="=",
-    all_bullets=False,
-    render_title=True,
-):
+    template: str,
+    issue_format: str | None,
+    fragments: Mapping[str, Mapping[str, Mapping[str, Sequence[str]]]],
+    definitions: Sequence[str],
+    underlines: Sequence[str],
+    wrap: bool,
+    versiondata: Mapping[str, str],
+    top_underline: str = "=",
+    all_bullets: bool = False,
+    render_title: bool = True,
+) -> str:
     """
     Render the fragments into a news file.
     """
 
     jinja_template = Template(template, trim_blocks=True)
 
-    data = OrderedDict()
+    data: dict[str, dict[str, dict[str, list[str]]]] = OrderedDict()
 
     for section_name, section_value in fragments.items():
 
@@ -271,7 +287,7 @@ def render_fragments(
 
     done = []
 
-    def get_indent(text):
+    def get_indent(text: str) -> str:
         # If bullets are not assumed and we wrap, the subsequent
         # indentation depends on whether or not this is a bullet point.
         # (it is probably usually best to disable wrapping in that case)
diff --git a/src/towncrier/_git.py b/src/towncrier/_git.py
index 54afbf38..c4360870 100644
--- a/src/towncrier/_git.py
+++ b/src/towncrier/_git.py
@@ -1,6 +1,8 @@
 # Copyright (c) Amber Brown, 2015
 # See LICENSE for details.
 
+from __future__ import annotations
+
 import os
 
 from subprocess import STDOUT, call, check_output
@@ -8,7 +10,7 @@
 import click
 
 
-def remove_files(fragment_filenames, answer_yes):
+def remove_files(fragment_filenames: list[str], answer_yes: bool) -> None:
     if not fragment_filenames:
         return
 
@@ -24,12 +26,12 @@ def remove_files(fragment_filenames, answer_yes):
         call(["git", "rm", "--quiet"] + fragment_filenames)
 
 
-def stage_newsfile(directory, filename):
+def stage_newsfile(directory: str, filename: str) -> None:
 
     call(["git", "add", os.path.join(directory, filename)])
 
 
-def get_remote_branches(base_directory):
+def get_remote_branches(base_directory: str) -> list[str]:
     output = check_output(
         ["git", "branch", "-r"], cwd=base_directory, encoding="utf-8", stderr=STDOUT
     )
@@ -37,7 +39,9 @@ def get_remote_branches(base_directory):
     return [branch.strip() for branch in output.strip().splitlines()]
 
 
-def list_changed_files_compared_to_branch(base_directory, compare_with):
+def list_changed_files_compared_to_branch(
+    base_directory: str, compare_with: str
+) -> list[str]:
     output = check_output(
         ["git", "diff", "--name-only", compare_with + "..."],
         cwd=base_directory,
diff --git a/src/towncrier/_project.py b/src/towncrier/_project.py
index c5e4676f..9c887ff0 100644
--- a/src/towncrier/_project.py
+++ b/src/towncrier/_project.py
@@ -6,14 +6,17 @@
 """
 
 
+from __future__ import annotations
+
 import sys
 
 from importlib import import_module
+from types import ModuleType
 
 from incremental import Version
 
 
-def _get_package(package_dir, package):
+def _get_package(package_dir: str, package: str) -> ModuleType:
 
     try:
         module = import_module(package)
@@ -35,7 +38,7 @@ def _get_package(package_dir, package):
     return module
 
 
-def get_version(package_dir, package):
+def get_version(package_dir: str, package: str) -> str:
 
     module = _get_package(package_dir, package)
 
@@ -60,7 +63,7 @@ def get_version(package_dir, package):
     )
 
 
-def get_project_name(package_dir, package):
+def get_project_name(package_dir: str, package: str) -> str:
 
     module = _get_package(package_dir, package)
 
@@ -76,3 +79,5 @@ def get_project_name(package_dir, package):
     if isinstance(version, Version):
         # Incremental has support for package names
         return version.package
+
+    raise TypeError(f"Unsupported type for __version__: {type(version)}")
diff --git a/src/towncrier/_settings/__init__.py b/src/towncrier/_settings/__init__.py
index 2eb48803..0f25f2f4 100644
--- a/src/towncrier/_settings/__init__.py
+++ b/src/towncrier/_settings/__init__.py
@@ -1,5 +1,7 @@
 """Subpackage to handle settings parsing."""
 
+from __future__ import annotations
+
 from towncrier._settings import load
 
 
diff --git a/src/towncrier/_settings/fragment_types.py b/src/towncrier/_settings/fragment_types.py
index 434b6475..a9b83676 100644
--- a/src/towncrier/_settings/fragment_types.py
+++ b/src/towncrier/_settings/fragment_types.py
@@ -1,19 +1,23 @@
+from __future__ import annotations
+
 import abc
 import collections as clt
 
+from typing import Any, Iterable, Mapping
+
 
 class BaseFragmentTypesLoader:
     """Base class to load fragment types."""
 
     __metaclass__ = abc.ABCMeta
 
-    def __init__(self, config):
+    def __init__(self, config: Mapping[str, Any]):
         """Initialize."""
         self.config = config
 
     @classmethod
-    def factory(cls, config):
-        fragment_types_class = DefaultFragmentTypesLoader
+    def factory(cls, config: Mapping[str, Any]) -> BaseFragmentTypesLoader:
+        fragment_types_class: type[BaseFragmentTypesLoader] = DefaultFragmentTypesLoader
         fragment_types = config.get("fragment", {})
         types_config = config.get("type", {})
         if fragment_types:
@@ -25,7 +29,7 @@ def factory(cls, config):
         return new
 
     @abc.abstractmethod
-    def load(self):
+    def load(self) -> Mapping[str, Mapping[str, Any]]:
         """Load fragment types."""
 
 
@@ -42,7 +46,7 @@ class DefaultFragmentTypesLoader(BaseFragmentTypesLoader):
         ]
     )
 
-    def load(self):
+    def load(self) -> Mapping[str, Mapping[str, Any]]:
         """Load default types."""
         return self._default_types
 
@@ -64,7 +68,7 @@ class ArrayFragmentTypesLoader(BaseFragmentTypesLoader):
 
     """
 
-    def load(self):
+    def load(self) -> Mapping[str, Mapping[str, Any]]:
         """Load types from toml array of mappings."""
 
         types = clt.OrderedDict()
@@ -105,14 +109,14 @@ class TableFragmentTypesLoader(BaseFragmentTypesLoader):
 
     """
 
-    def __init__(self, config):
+    def __init__(self, config: Mapping[str, Mapping[str, Any]]):
         """Initialize."""
         self.config = config
         self.fragment_options = config.get("fragment", {})
 
-    def load(self):
+    def load(self) -> Mapping[str, Mapping[str, Any]]:
         """Load types from nested mapping."""
-        fragment_types = self.fragment_options.keys()
+        fragment_types: Iterable[str] = self.fragment_options.keys()
         fragment_types = sorted(fragment_types)
         custom_types_sequence = [
             (fragment_type, self._load_options(fragment_type))
@@ -121,7 +125,7 @@ def load(self):
         types = clt.OrderedDict(custom_types_sequence)
         return types
 
-    def _load_options(self, fragment_type):
+    def _load_options(self, fragment_type: str) -> Mapping[str, Any]:
         """Load fragment options."""
         capitalized_fragment_type = fragment_type.capitalize()
         options = self.fragment_options.get(fragment_type, {})
diff --git a/src/towncrier/_settings/load.py b/src/towncrier/_settings/load.py
index f4335af8..534a638f 100644
--- a/src/towncrier/_settings/load.py
+++ b/src/towncrier/_settings/load.py
@@ -1,9 +1,12 @@
 # Copyright (c) Amber Brown, 2015
 # See LICENSE for details.
 
+from __future__ import annotations
+
 import os
 
 from collections import OrderedDict
+from typing import Any, Mapping
 
 import pkg_resources
 import tomli
@@ -12,7 +15,7 @@
 
 
 class ConfigError(Exception):
-    def __init__(self, *args, **kwargs):
+    def __init__(self, *args: str, **kwargs: str):
         self.failing_option = kwargs.get("failing_option")
         super().__init__(*args)
 
@@ -23,20 +26,22 @@ def __init__(self, *args, **kwargs):
 _underlines = ["=", "-", "~"]
 
 
-def load_config_from_options(directory, config):
-    if config is None:
+def load_config_from_options(
+    directory: str | None, config_path: str | None
+) -> tuple[str, Mapping[str, Any]]:
+    if config_path is None:
         if directory is None:
             directory = os.getcwd()
 
         base_directory = os.path.abspath(directory)
         config = load_config(base_directory)
     else:
-        config = os.path.abspath(config)
+        config_path = os.path.abspath(config_path)
         if directory:
             base_directory = os.path.abspath(directory)
         else:
-            base_directory = os.path.dirname(config)
-        config = load_config_from_file(os.path.dirname(config), config)
+            base_directory = os.path.dirname(config_path)
+        config = load_config_from_file(os.path.dirname(config_path), config_path)
 
     if config is None:
         raise ConfigError(f"No configuration file found.\nLooked in: {base_directory}")
@@ -44,7 +49,7 @@ def load_config_from_options(directory, config):
     return base_directory, config
 
 
-def load_config(directory):
+def load_config(directory: str) -> Mapping[str, Any] | None:
 
     towncrier_toml = os.path.join(directory, "towncrier.toml")
     pyproject_toml = os.path.join(directory, "pyproject.toml")
@@ -59,14 +64,14 @@ def load_config(directory):
     return load_config_from_file(directory, config_file)
 
 
-def load_config_from_file(directory, config_file):
+def load_config_from_file(directory: str, config_file: str) -> Mapping[str, Any]:
     with open(config_file, "rb") as conffile:
         config = tomli.load(conffile)
 
     return parse_toml(directory, config)
 
 
-def parse_toml(base_path, config):
+def parse_toml(base_path: str, config: Mapping[str, Any]) -> Mapping[str, Any]:
     if "tool" not in config:
         raise ConfigError("No [tool.towncrier] section.", failing_option="all")
 
diff --git a/src/towncrier/_shell.py b/src/towncrier/_shell.py
index fd916ab4..0efe1b69 100644
--- a/src/towncrier/_shell.py
+++ b/src/towncrier/_shell.py
@@ -7,6 +7,8 @@
 Each sub-command has its separate CLI definition andd help messages.
 """
 
+from __future__ import annotations
+
 import click
 
 from click_default_group import DefaultGroup
@@ -19,7 +21,7 @@
 
 @click.group(cls=DefaultGroup, default="build", default_if_no_args=True)
 @click.version_option(__version__.public())
-def cli():
+def cli() -> None:
     """
     Towncrier is a utility to produce useful, summarised news files for your project.
     Rather than reading the Git history as some newer tools to produce it, or having
diff --git a/src/towncrier/_writer.py b/src/towncrier/_writer.py
index e58535b6..92119114 100644
--- a/src/towncrier/_writer.py
+++ b/src/towncrier/_writer.py
@@ -6,12 +6,19 @@
 affecting existing content.
 """
 
+from __future__ import annotations
+
 from pathlib import Path
 
 
 def append_to_newsfile(
-    directory, filename, start_string, top_line, content, single_file
-):
+    directory: str,
+    filename: str,
+    start_string: str,
+    top_line: str,
+    content: str,
+    single_file: bool,
+) -> None:
     """
     Write *content* to *directory*/*filename* behind *start_string*.
 
@@ -43,7 +50,9 @@ def append_to_newsfile(
             f.write(f"\n\n{prev_body}")
 
 
-def _figure_out_existing_content(news_file, start_string, single_file):
+def _figure_out_existing_content(
+    news_file: Path, start_string: str, single_file: bool
+) -> tuple[str, str]:
     """
     Try to read *news_file* and split it into header (everything before
     *start_string*) and the old body (everything after *start_string*).
diff --git a/src/towncrier/build.py b/src/towncrier/build.py
index a59b13cf..5c4a012c 100644
--- a/src/towncrier/build.py
+++ b/src/towncrier/build.py
@@ -6,6 +6,8 @@
 """
 
 
+from __future__ import annotations
+
 import os
 import sys
 
@@ -16,15 +18,11 @@
 from ._builder import find_fragments, render_fragments, split_fragments
 from ._git import remove_files, stage_newsfile
 from ._project import get_project_name, get_version
-from ._settings import (
-    ConfigError,
-    config_option_help,
-    load_config_from_options,
-)
+from ._settings import ConfigError, config_option_help, load_config_from_options
 from ._writer import append_to_newsfile
 
 
-def _get_date():
+def _get_date() -> str:
     return date.today().isoformat()
 
 
@@ -74,14 +72,14 @@ def _get_date():
     help="Do not ask for confirmation to remove news fragments.",
 )
 def _main(
-    draft,
-    directory,
-    config_file,
-    project_name,
-    project_version,
-    project_date,
-    answer_yes,
-):
+    draft: bool,
+    directory: str | None,
+    config_file: str | None,
+    project_name: str | None,
+    project_version: str | None,
+    project_date: str | None,
+    answer_yes: bool,
+) -> None:
     """
     Build a combined news file from news fragment.
     """
@@ -101,14 +99,14 @@ def _main(
 
 
 def __main(
-    draft,
-    directory,
-    config_file,
-    project_name,
-    project_version,
-    project_date,
-    answer_yes,
-):
+    draft: bool,
+    directory: str | None,
+    config_file: str | None,
+    project_name: str | None,
+    project_version: str | None,
+    project_date: str | None,
+    answer_yes: bool,
+) -> None:
     """
     The main entry point.
     """
@@ -132,13 +130,13 @@ def __main(
         )
         fragment_directory = "newsfragments"
 
-    fragments, fragment_filenames = find_fragments(
+    fragment_contents, fragment_filenames = find_fragments(
         fragment_base_directory, config["sections"], fragment_directory, definitions
     )
 
     click.echo("Rendering news fragments...", err=to_err)
     fragments = split_fragments(
-        fragments, definitions, all_bullets=config["all_bullets"]
+        fragment_contents, definitions, all_bullets=config["all_bullets"]
     )
 
     if project_version is None:
diff --git a/src/towncrier/check.py b/src/towncrier/check.py
index deb86fc9..9dae3ba7 100644
--- a/src/towncrier/check.py
+++ b/src/towncrier/check.py
@@ -2,10 +2,13 @@
 # See LICENSE for details.
 
 
+from __future__ import annotations
+
 import os
 import sys
 
 from subprocess import CalledProcessError
+from typing import Container
 from warnings import warn
 
 import click
@@ -15,7 +18,7 @@
 from ._settings import config_option_help, load_config_from_options
 
 
-def _get_default_compare_branch(branches):
+def _get_default_compare_branch(branches: Container[str]) -> str | None:
     if "origin/main" in branches:
         return "origin/main"
     if "origin/master" in branches:
@@ -54,16 +57,18 @@ def _get_default_compare_branch(branches):
     metavar="FILE_PATH",
     help=config_option_help,
 )
-def _main(compare_with, directory, config):
+def _main(compare_with: str | None, directory: str | None, config: str | None) -> None:
     """
     Check for new fragments on a branch.
     """
-    return __main(compare_with, directory, config)
+    __main(compare_with, directory, config)
 
 
-def __main(comparewith, directory, config):
+def __main(
+    comparewith: str | None, directory: str | None, config_path: str | None
+) -> None:
 
-    base_directory, config = load_config_from_options(directory, config)
+    base_directory, config = load_config_from_options(directory, config_path)
 
     if comparewith is None:
         comparewith = _get_default_compare_branch(
diff --git a/src/towncrier/create.py b/src/towncrier/create.py
index ce37fc49..a075f48c 100644
--- a/src/towncrier/create.py
+++ b/src/towncrier/create.py
@@ -5,6 +5,8 @@
 Create a new fragment.
 """
 
+from __future__ import annotations
+
 import os
 
 import click
@@ -41,7 +43,14 @@
     help="Sets the content of the new fragment.",
 )
 @click.argument("filename")
-def _main(ctx, directory, config, filename, edit, content):
+def _main(
+    ctx: click.Context,
+    directory: str | None,
+    config: str | None,
+    filename: str,
+    edit: bool,
+    content: str,
+) -> None:
     """
     Create a new news fragment.
 
@@ -56,14 +65,21 @@ def _main(ctx, directory, config, filename, edit, content):
     * .removal - a deprecation or removal of public API,
     * .misc - a ticket has been closed, but it is not of interest to users.
     """
-    return __main(ctx, directory, config, filename, edit, content)
+    __main(ctx, directory, config, filename, edit, content)
 
 
-def __main(ctx, directory, config, filename, edit, content):
+def __main(
+    ctx: click.Context,
+    directory: str | None,
+    config_path: str | None,
+    filename: str,
+    edit: bool,
+    content: str,
+) -> None:
     """
     The main entry point.
     """
-    base_directory, config = load_config_from_options(directory, config)
+    base_directory, config = load_config_from_options(directory, config_path)
 
     definitions = config["types"] or []
     if len(filename.split(".")) < 2 or (
@@ -98,11 +114,11 @@ def __main(ctx, directory, config, filename, edit, content):
         raise click.ClickException(f"{segment_file} already exists")
 
     if edit:
-        content = _get_news_content_from_user(content)
-
-    if content is None:
-        click.echo("Abort creating news fragment.")
-        ctx.exit(1)
+        edited_content = _get_news_content_from_user(content)
+        if edited_content is None:
+            click.echo("Abort creating news fragment.")
+            ctx.exit(1)
+        content = edited_content
 
     with open(segment_file, "w") as f:
         f.write(content)
@@ -110,7 +126,7 @@ def __main(ctx, directory, config, filename, edit, content):
     click.echo(f"Created news fragment at {segment_file}")
 
 
-def _get_news_content_from_user(message):
+def _get_news_content_from_user(message: str) -> str | None:
     initial_content = (
         "# Please write your news content. When finished, save the file.\n"
         "# In order to abort, exit without saving.\n"
diff --git a/src/towncrier/newsfragments/421.misc b/src/towncrier/newsfragments/421.misc
new file mode 100644
index 00000000..e69de29b
diff --git a/src/towncrier/test/test_project.py b/src/towncrier/test/test_project.py
index 6c9b4657..4d3531ce 100644
--- a/src/towncrier/test/test_project.py
+++ b/src/towncrier/test/test_project.py
@@ -8,7 +8,7 @@
 
 from twisted.trial.unittest import TestCase
 
-from .._project import get_version
+from .._project import get_project_name, get_version
 
 
 class VersionFetchingTests(TestCase):
@@ -40,6 +40,21 @@ def test_tuple(self):
         version = get_version(temp, "mytestproja")
         self.assertEqual(version, "1.3.12")
 
+    def test_unknown_type(self):
+        """
+        A __version__ of unknown type will lead to an exception.
+        """
+        temp = self.mktemp()
+        os.makedirs(temp)
+        os.makedirs(os.path.join(temp, "mytestprojb"))
+
+        with open(os.path.join(temp, "mytestprojb", "__init__.py"), "w") as f:
+            f.write("__version__ = object()")
+
+        self.assertRaises(Exception, get_version, temp, "mytestprojb")
+
+        self.assertRaises(TypeError, get_project_name, temp, "mytestprojb")
+
     def test_import_fails(self):
         """
         An exception is raised when getting the version failed due to missing Python package files.
diff --git a/tox.ini b/tox.ini
index b569d967..bcc85c1c 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,5 +1,5 @@
 [tox]
-envlist = pre-commit, {pypy37,pypy38,py37,py38,py39,py310}-tests, check-manifest, check-newsfragment
+envlist = pre-commit, {pypy37,pypy38,py37,py38,py39,py310}-tests, check-manifest, check-newsfragment, typecheck
 isolated_build=true
 skip_missing_envs = true
 
@@ -35,6 +35,15 @@ commands =
    coverage combine -a
    coverage report
 
+[testenv:typecheck]
+deps =
+   mypy
+   # 2022-09-04: There is no release yet which includes type annotations.
+   incremental @ git+https://github.com/twisted/incremental.git@5845557ab9
+   types-setuptools
+commands =
+   mypy src/
+
 [testenv:build]
 allowlist_externals =
    bash