Fix a bug in the joined_rooms admin API (#8643)

If the user was not in any rooms then the API returned the same error
as if the user did not exist.

1 files changed, 4 insertions, 3 deletions

diff --git a/synapse/rest/admin/users.py b/synapse/rest/admin/users.py
index 8efefbc0a0..e71d9b0e1c 100644
--- a/synapse/rest/admin/users.py
+++ b/synapse/rest/admin/users.py
@@ -702,9 +702,10 @@ class UserMembershipRestServlet(RestServlet):
         if not self.is_mine(UserID.from_string(user_id)):
             raise SynapseError(400, "Can only lookup local users")
 
-        room_ids = await self.store.get_rooms_for_user(user_id)
-        if not room_ids:
-            raise NotFoundError("User not found")
+        user = await self.store.get_user_by_id(user_id)
+        if user is None:
+            raise NotFoundError("Unknown user")
 
+        room_ids = await self.store.get_rooms_for_user(user_id)
         ret = {"joined_rooms": list(room_ids), "total": len(room_ids)}
         return 200, ret