diff --git a/segment/tests/test_video_segment.py b/segment/tests/test_video_segment.py
index 1b75156..79e414c 100644
--- a/segment/tests/test_video_segment.py
+++ b/segment/tests/test_video_segment.py
@@ -26,7 +26,9 @@
     def test_init(self):
         """Test segment initialises correctly."""
         super().test_init()
-        self.assertEqual(self.segment._temp_frame_file, "")
+        self.assertEqual(
+            self.segment._temp_frame_file, "",
+            msg="temp frame file = ")
     
     # Tricky to test get_last_frame_number() and generate_frame()
     # because they use pexpect.