diff --git a/Lib/test/test_io.py b/Lib/test/test_io.py
index ebaf796..26c5dfe 100644
--- a/Lib/test/test_io.py
+++ b/Lib/test/test_io.py
@@ -690,6 +690,16 @@
         self.assertEqual(stream.readinto(buffer), 5)
         self.assertEqual(buffer.tobytes(), b"12345")
 
+    def test_close_assert(self):
+        class R(self.IOBase):
+            def __setattr__(self, name, value):
+                pass
+            def flush(self):
+                raise OSError()
+        f = R()
+        # This would cause an assertion failure.
+        self.assertRaises(OSError, f.close)
+
 
 class CIOTest(IOTest):