diff --git a/.github/workflows/testsuite.yml b/.github/workflows/testsuite.yml
index be90065e..840f4526 100644
--- a/.github/workflows/testsuite.yml
+++ b/.github/workflows/testsuite.yml
@@ -114,7 +114,7 @@ jobs:
         run: |
           pip install -r requirements.txt
           pip install -U pytest==${{ matrix.pytest-version }}
-          pip install opentimelineio
+          pip install opentimelineio undefined
           pip install -e .
           if [[ '${{ matrix.pytest-version }}' == '4.0.2' ]]; then
              pip install -U attrs==19.1.0
diff --git a/CHANGES.md b/CHANGES.md
index cac49c56..3aa409eb 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -1,6 +1,11 @@
 # pyfakefs Release Notes
 The released versions correspond to PyPI releases.
 
+## Unreleased
+
+### Fixes
+* fixes handling of unhashable modules which cannot be cached (see [#923](../../issues/923))
+
 ## [Version 5.3.2](https://pypi.python.org/pypi/pyfakefs/5.3.2) (2023-11-30)
 Bugfix release.
 
diff --git a/pyfakefs/fake_filesystem_unittest.py b/pyfakefs/fake_filesystem_unittest.py
index c6495e0c..271791d5 100644
--- a/pyfakefs/fake_filesystem_unittest.py
+++ b/pyfakefs/fake_filesystem_unittest.py
@@ -805,7 +805,11 @@ def _find_modules(self) -> None:
                 # see https://github.com/pytest-dev/py/issues/73
                 # and any other exception triggered by inspect.ismodule
                 if self.use_cache:
-                    self.__class__.CACHED_MODULES.add(module)
+                    try:
+                        self.__class__.CACHED_MODULES.add(module)
+                    except TypeError:
+                        # unhashable module - don't cache it
+                        pass
                 continue
             skipped = module in self.SKIPMODULES or any(
                 [sn.startswith(module.__name__) for sn in self._skip_names]
diff --git a/pyfakefs/pytest_tests/pytest_fixture_test.py b/pyfakefs/pytest_tests/pytest_fixture_test.py
index 27ea9081..2b158980 100644
--- a/pyfakefs/pytest_tests/pytest_fixture_test.py
+++ b/pyfakefs/pytest_tests/pytest_fixture_test.py
@@ -13,6 +13,7 @@
 # Example for a test using a custom pytest fixture with an argument to Patcher
 
 import pytest
+import undefined
 
 import pyfakefs.pytest_tests.example as example
 from pyfakefs.fake_filesystem_unittest import Patcher
@@ -41,6 +42,12 @@ def test_example_file_passing_using_patcher():
         check_that_example_file_is_in_fake_fs()
 
 
+def test_undefined(fs):
+    # regression test for #923
+    with pytest.raises(NotImplementedError):
+        print(undefined)
+
+
 def check_that_example_file_is_in_fake_fs():
     with open(example.EXAMPLE_FILE) as file:
         assert file.read() == "stuff here"