diff --git a/src/backend/optimizer/plan/planner.c b/src/backend/optimizer/plan/planner.c
index 247f7d4625..1a94b58f8b 100644
--- a/src/backend/optimizer/plan/planner.c
+++ b/src/backend/optimizer/plan/planner.c
@@ -6375,8 +6375,11 @@ plan_create_index_workers(Oid tableOid, Oid indexOid)
 	double		reltuples;
 	double		allvisfrac;
 
-	/* Return immediately when parallelism disabled */
-	if (max_parallel_maintenance_workers == 0)
+	/*
+	 * We don't allow performing parallel operation in standalone backend or
+	 * when parallelism is disabled.
+	 */
+	if (!IsUnderPostmaster || max_parallel_maintenance_workers == 0)
 		return 0;
 
 	/* Set up largely-dummy planner state */