diff --git a/src/backend/executor/execMain.c b/src/backend/executor/execMain.c
index 99f7a049930..315a555dcff 100644
--- a/src/backend/executor/execMain.c
+++ b/src/backend/executor/execMain.c
@@ -807,6 +807,10 @@ InitPlan(QueryDesc *queryDesc, int eflags)
 		if (rc->isParent)
 			continue;
 
+		/*
+		 * If you change the conditions under which rel locks are acquired
+		 * here, be sure to adjust ExecOpenScanRelation to match.
+		 */
 		switch (rc->markType)
 		{
 			case ROW_MARK_EXCLUSIVE:
diff --git a/src/backend/executor/execUtils.c b/src/backend/executor/execUtils.c
index 8b341dae4ed..c4d048f4216 100644
--- a/src/backend/executor/execUtils.c
+++ b/src/backend/executor/execUtils.c
@@ -820,7 +820,9 @@ ExecOpenScanRelation(EState *estate, Index scanrelid, int eflags)
 		{
 			ExecRowMark *erm = lfirst(l);
 
-			if (erm->rti == scanrelid)
+			/* Keep this check in sync with InitPlan! */
+			if (erm->rti == scanrelid &&
+				erm->relation != NULL)
 			{
 				lockmode = NoLock;
 				break;