diff --git a/openhtf/util/test.py b/openhtf/util/test.py
index 4f882580..54cabb7b 100644
--- a/openhtf/util/test.py
+++ b/openhtf/util/test.py
@@ -954,6 +954,7 @@ def assertAttachment(self,
 
   def get_diagnoses_store(self):
     self.assertIsNotNone(self.last_test_state)
+    assert self.last_test_state is not None
     return self.last_test_state.diagnoses_manager.store
 
   @classmethod