diff --git a/synapse/config/cas.py b/synapse/config/cas.py
index c636a34e0a30..6e2d9addbf4c 100644
--- a/synapse/config/cas.py
+++ b/synapse/config/cas.py
@@ -42,7 +42,10 @@ def read_config(self, config: JsonDict, **kwargs: Any) -> None:
             self.cas_service_url = public_baseurl + "_matrix/client/r0/login/cas/ticket"
 
             self.cas_protocol_version = cas_config.get("protocol_version")
-            if self.cas_protocol_version is not None and self.cas_protocol_version not in [1, 2, 3]:
+            if (
+                self.cas_protocol_version is not None
+                and self.cas_protocol_version not in [1, 2, 3]
+            ):
                 raise ConfigError(
                     "Unsupported CAS protocol version %s (only versions 1, 2, 3 are supported)"
                     % (self.cas_protocol_version,),