diff --git a/mypy/test/testdmypy.py b/mypy/test/testdmypy.py
index 4be4ed8259d5..dedceccabd45 100644
--- a/mypy/test/testdmypy.py
+++ b/mypy/test/testdmypy.py
@@ -30,7 +30,7 @@
         'check-newtype.test',
     ]
 else:
-    dmypy_files = []
+    dmypy_files = []  # type: List[str]
 
 
 class TypeCheckSuite(DataSuite):