1 files changed, 3 insertions, 0 deletions
diff --git a/synapse/handlers/cas.py b/synapse/handlers/cas.py
index 7163af8004..fc467bc7c1 100644
--- a/synapse/handlers/cas.py
+++ b/synapse/handlers/cas.py
@@ -130,6 +130,9 @@ class CasHandler:
         except PartialDownloadError as pde:
             # Twisted raises this error if the connection is closed,
             # even if that's being used old-http style to signal end-of-data
+            # Assertion is for mypy's benefit. Error.response is Optional[bytes],
+            # but a PartialDownloadError should always have a non-None response.
+            assert pde.response is not None
             body = pde.response
         except HttpResponseException as e:
             description = (