diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
index 777f9b2..201b221 100644
--- a/.github/workflows/main.yml
+++ b/.github/workflows/main.yml
@@ -24,11 +24,12 @@ jobs:
         python tests/update_submodules.py
     - name: Install Python package
       env:
-        # See https://github.com/pypa/wheel/issues/507:
-        PYTHONWARNINGS: error,ignore:pkg_resources is deprecated:DeprecationWarning
+        PYTHONWARNINGS: error,default::DeprecationWarning
       run: |
         python -m pip install .
     - name: Install test dependencies
+      env:
+        PYTHONWARNINGS: error,default::DeprecationWarning
       run: |
         python -m pip install -r tests/requirements.txt
     - name: Run pytest
diff --git a/src/sphinx_last_updated_by_git.py b/src/sphinx_last_updated_by_git.py
index d4b14d7..baad12b 100644
--- a/src/sphinx_last_updated_by_git.py
+++ b/src/sphinx_last_updated_by_git.py
@@ -276,6 +276,11 @@ def _builder_inited(app):
 
 def _source_read(app, docname, source):
     env = app.env
+    if docname not in env.found_docs:
+        # Since Sphinx 7.2, "docname" can be None or a relative path
+        # to a file included with the "include" directive.
+        # We are only interested in actual source documents.
+        return
     assert docname not in env.git_last_updated
     env.git_last_updated[docname] = None