diff --git a/synapse/handlers/e2e_room_keys.py b/synapse/handlers/e2e_room_keys.py
index a953a7fe0..cad38d2bd 100644
--- a/synapse/handlers/e2e_room_keys.py
+++ b/synapse/handlers/e2e_room_keys.py
@@ -209,8 +209,9 @@ class E2eRoomKeysHandler(object):
                 for session_id, room_key in iteritems(room["sessions"]):
                     if not isinstance(room_key["is_verified"], bool):
                         msg = (
-                            "is_verified must be a boolean in keys for room %s"
-                            % room_id
+                            "is_verified must be a boolean in keys for session %s in"
+                            "room %s"
+                            % (session_id, room_id)
                         )
                         raise SynapseError(400, msg, Codes.INVALID_PARAM)