diff --git a/synapse/rest/client/v2_alpha/shared_rooms.py b/synapse/rest/client/v2_alpha/shared_rooms.py
index ca86a9a910..86feec2145 100644
--- a/synapse/rest/client/v2_alpha/shared_rooms.py
+++ b/synapse/rest/client/v2_alpha/shared_rooms.py
@@ -43,7 +43,7 @@ class UserSharedRoomsServlet(RestServlet):
             raise SynapseError(
                 code=400,
                 msg="'user_id' must not be the authenticated user",
-                errcode=Codes.BAD_JSON,
+                errcode=Codes.FORBIDDEN,
             )
 
         rooms = await self.store.get_rooms_in_common_for_users(