--- a/src/backend/commands/repack.c
+++ b/src/backend/commands/repack.c
@@ -577,8 +577,9 @@
 		ereport(ERROR,
 				errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
 		/*- translator: first %s is name of a SQL command, eg. REPACK */
-				errmsg("cannot execute %s on a shared catalog",
-					   RepackCommandAsString(cmd)));
+				errmsg("cannot execute %s USING INDEX on a shared catalog",
+					   RepackCommandAsString(cmd)),
+				errdetail("Marking an index as clustered on a shared catalog would only take effect in the current database."));
 
 	/*
 	 * The CONCURRENTLY case should have been rejected earlier because it does