diff --git a/Lib/test/test_threaded_import.py b/Lib/test/test_threaded_import.py
index 0528b13..93bfb2a 100644
--- a/Lib/test/test_threaded_import.py
+++ b/Lib/test/test_threaded_import.py
@@ -68,6 +68,7 @@
         # Simulate some thread-unsafe behaviour. If calls to find_module()
         # are properly serialized, `x` will end up the same as `numcalls`.
         # Otherwise not.
+        assert imp.lock_held()
         with self.lock:
             self.numcalls += 1
         x = self.x