diff --git a/flask/cli.py b/flask/cli.py
index dca728ce..0e2aec6e 100644
--- a/flask/cli.py
+++ b/flask/cli.py
@@ -132,6 +132,8 @@ def _called_with_wrong_args(factory):
         # didn't reach the factory
         return True
     finally:
+        # explicitly delete tb as it is circular referenced
+        # https://docs.python.org/2/library/sys.html#sys.exc_info
         del tb