diff --git a/.changesets/fix-no-implicit-conversion-of-pathname-into-string-error.md b/.changesets/fix-no-implicit-conversion-of-pathname-into-string-error.md
new file mode 100644
index 00000000..0e0492c1
--- /dev/null
+++ b/.changesets/fix-no-implicit-conversion-of-pathname-into-string-error.md
@@ -0,0 +1,6 @@
+---
+bump: patch
+type: fix
+---
+
+Fix 'no implicit conversion of Pathname into String' error when parsing backtrace lines of error causes in Rails apps.
diff --git a/lib/appsignal/config.rb b/lib/appsignal/config.rb
index 42399cdd..ee85eb39 100644
--- a/lib/appsignal/config.rb
+++ b/lib/appsignal/config.rb
@@ -212,7 +212,7 @@ def initialize(
       root_path,
       env
     )
-      @root_path = root_path
+      @root_path = root_path.to_s
       @config_file_error = false
       @config_file = config_file
       @valid = false
diff --git a/spec/lib/appsignal/config_spec.rb b/spec/lib/appsignal/config_spec.rb
index 40755bd0..e3106725 100644
--- a/spec/lib/appsignal/config_spec.rb
+++ b/spec/lib/appsignal/config_spec.rb
@@ -371,6 +371,15 @@ def on_load
     end
   end
 
+  context "when root path is Pathname instance" do
+    let(:config) { described_class.new(Pathname.new("/path"), "production") }
+
+    it "converts it to a String" do
+      expect(config.root_path).to eq("/path")
+      expect(config.root_path).to be_instance_of(String)
+    end
+  end
+
   context "without config file" do
     let(:config) { described_class.new(tmp_dir, "production") }