diff --git a/numcodecs/abc.py b/numcodecs/abc.py
index 5aba8c25..a57c13e2 100644
--- a/numcodecs/abc.py
+++ b/numcodecs/abc.py
@@ -29,14 +29,13 @@
 """
 
 from abc import ABC, abstractmethod
-from typing import Optional
+from typing import ClassVar
 
 
 class Codec(ABC):
     """Codec abstract base class."""
 
-    # override in sub-class
-    codec_id: Optional[str] = None
+    codec_id: ClassVar[str]
     """Codec identifier."""
 
     @abstractmethod
diff --git a/numcodecs/tests/test_registry.py b/numcodecs/tests/test_registry.py
index 93ed8f03..131b8773 100644
--- a/numcodecs/tests/test_registry.py
+++ b/numcodecs/tests/test_registry.py
@@ -34,8 +34,8 @@ def test_all_classes_registered():
         if (
             inspect.isclass(obj)
             and issubclass(obj, numcodecs.abc.Codec)
+            and hasattr(obj, 'codec_id')
             and obj.codec_id not in numcodecs.registry.codec_registry
-            and obj.codec_id is not None  # remove `None`
         )
     }