diff --git a/src/backend/optimizer/plan/planner.c b/src/backend/optimizer/plan/planner.c
index 00db38e90b..173f0d21fe 100644
--- a/src/backend/optimizer/plan/planner.c
+++ b/src/backend/optimizer/plan/planner.c
@@ -6093,11 +6093,13 @@ plan_create_index_workers(Oid tableOid, Oid indexOid)
 	/*
 	 * Determine if it's safe to proceed.
 	 *
-	 * Currently, parallel workers can't access the leader's temporary tables.
-	 * Furthermore, any index predicate or index expressions must be parallel
-	 * safe.
+	 * Currently, parallel workers can't access the leader's temporary tables,
+	 * or the leader's relmapper.c state, which is needed for builds on mapped
+	 * relations.  Furthermore, any index predicate or index expressions must
+	 * be parallel safe.
 	 */
 	if (heap->rd_rel->relpersistence == RELPERSISTENCE_TEMP ||
+		RelationIsMapped(heap) ||
 		!is_parallel_safe(root, (Node *) RelationGetIndexExpressions(index)) ||
 		!is_parallel_safe(root, (Node *) RelationGetIndexPredicate(index)))
 	{
diff --git a/src/backend/utils/cache/relmapper.c b/src/backend/utils/cache/relmapper.c
index 99d095f2df..e3a5e4e386 100644
--- a/src/backend/utils/cache/relmapper.c
+++ b/src/backend/utils/cache/relmapper.c
@@ -263,13 +263,17 @@ RelationMapUpdateMap(Oid relationId, Oid fileNode, bool shared,
 	else
 	{
 		/*
-		 * We don't currently support map changes within subtransactions. This
-		 * could be done with more bookkeeping infrastructure, but it doesn't
-		 * presently seem worth it.
+		 * We don't currently support map changes within subtransactions, and
+		 * parallel workers must avoid relying on mapping state, since it
+		 * isn't propagated from the leader.  This could be done with more
+		 * bookkeeping infrastructure, but it doesn't presently seem worth it.
 		 */
 		if (GetCurrentTransactionNestLevel() > 1)
 			elog(ERROR, "cannot change relation mapping within subtransaction");
 
+		if (IsInParallelMode())
+			elog(ERROR, "cannot change relation mapping in parallel mode");
+
 		if (immediate)
 		{
 			/* Make it active, but only locally */