1 files changed, 3 insertions, 0 deletions
diff --git a/synapse/handlers/ui_auth/checkers.py b/synapse/handlers/ui_auth/checkers.py
index a744d68c64..332edcca24 100644
--- a/synapse/handlers/ui_auth/checkers.py
+++ b/synapse/handlers/ui_auth/checkers.py
@@ -119,6 +119,9 @@ class RecaptchaAuthChecker(UserInteractiveAuthChecker):
         except PartialDownloadError as pde:
             # Twisted is silly
             data = pde.response
+            # For mypy's benefit. A general Error.response is Optional[bytes], but
+            # a PartialDownloadError.response should be bytes AFAICS.
+            assert data is not None
             resp_body = json_decoder.decode(data.decode("utf-8"))
 
         if "success" in resp_body: