diff --git a/src/sql/engine/set/ob_merge_set_op.cpp b/src/sql/engine/set/ob_merge_set_op.cpp
index 4dce9193eae938e93c78ce6c6864d8701ddc28b7..7d0d09e156a9cd3477a5ba5cd043fa25a5560e3a 100644
--- a/src/sql/engine/set/ob_merge_set_op.cpp
+++ b/src/sql/engine/set/ob_merge_set_op.cpp
@@ -368,7 +368,7 @@ int ObMergeSetOp::locate_next_left_inside(ObOperator &child_op,
                 && OB_FAIL(cmp_(*last_row_.store_row_, child_op.get_spec().output_,
                                 curr_idx, eval_ctx_, cmp))) {
         LOG_WARN("failed to compare row", K(ret));
-      } else if (last_idx < 0) {
+      } else if (nullptr == last_row_.store_row_ && last_idx < 0) {
         ret = OB_ERR_UNEXPECTED;
         LOG_WARN("get wrong last idx", K(ret));
       } else if (nullptr == last_row_.store_row_