diff --git a/src/backend/access/nbtree/nbtsort.c b/src/backend/access/nbtree/nbtsort.c
index bc00799b96..dab41ea298 100644
--- a/src/backend/access/nbtree/nbtsort.c
+++ b/src/backend/access/nbtree/nbtsort.c
@@ -1257,14 +1257,6 @@ _bt_begin_parallel(BTBuildState *buildstate, bool isconcurrent, int request)
 	pcxt = CreateParallelContext("postgres", "_bt_parallel_build_main",
 								 request, true);
 
-	/* If no DSM segment was available, back out (do serial build) */
-	if (pcxt->seg == NULL)
-	{
-		DestroyParallelContext(pcxt);
-		ExitParallelMode();
-		return;
-	}
-
 	scantuplesortstates = leaderparticipates ? request + 1 : request;
 
 	/*
@@ -1308,6 +1300,16 @@ _bt_begin_parallel(BTBuildState *buildstate, bool isconcurrent, int request)
 	/* Everyone's had a chance to ask for space, so now create the DSM */
 	InitializeParallelDSM(pcxt);
 
+	/* If no DSM segment was available, back out (do serial build) */
+	if (pcxt->seg == NULL)
+	{
+		if (IsMVCCSnapshot(snapshot))
+			UnregisterSnapshot(snapshot);
+		DestroyParallelContext(pcxt);
+		ExitParallelMode();
+		return;
+	}
+
 	/* Store shared build state, for which we reserved space */
 	btshared = (BTShared *) shm_toc_allocate(pcxt->toc, estbtshared);
 	/* Initialize immutable state */