diff --git a/tests/test_image.py b/tests/test_image.py
index 34380c802e8d9a476a1b004d660b8a28e06acac7..47a6ad4ce74461b88c5589af4c86221a26b16783 100644
--- a/tests/test_image.py
+++ b/tests/test_image.py
@@ -39,8 +39,12 @@ except ImportError:
 try:
     import indexed_gzip as igzip
 except ImportError:
+
+    class MockError(Exception):
+        pass
+
     igzip           = mock.MagicMock()
-    igzip.ZranError = mock.MagicMock()
+    igzip.ZranError = MockError
 
 
 def make_image(filename=None,