diff --git a/clientapi/routing/joinroom.go b/clientapi/routing/joinroom.go
index 578aaec56..64ba59137 100644
--- a/clientapi/routing/joinroom.go
+++ b/clientapi/routing/joinroom.go
@@ -102,6 +102,10 @@ func JoinRoomByIDOrAlias(
 			JSON: jsonerror.Unknown("The room join will continue in the background."),
 		}
 	case result := <-done:
+		if !result.Is2xx() {
+			// The only error code in the spec for CS /join is 403.
+			result.Code = http.StatusForbidden
+		}
 		return result
 	}
 }