diff --git a/Lib/importlib/test/extension/test_case_sensitivity.py b/Lib/importlib/test/extension/test_case_sensitivity.py
index 39e6fac..3387e70 100644
--- a/Lib/importlib/test/extension/test_case_sensitivity.py
+++ b/Lib/importlib/test/extension/test_case_sensitivity.py
@@ -32,6 +32,8 @@
 
 
 def test_main():
+    if ext_util.FILENAME is None:
+        return
     support.run_unittest(ExtensionModuleCaseSensitivityTest)