diff --git a/src/tools/db-upgrade/NebulaKeyUtilsV3.cpp b/src/tools/db-upgrade/NebulaKeyUtilsV3.cpp
index f6105094f4f..e7e7033e389 100644
--- a/src/tools/db-upgrade/NebulaKeyUtilsV3.cpp
+++ b/src/tools/db-upgrade/NebulaKeyUtilsV3.cpp
@@ -15,7 +15,7 @@ std::string NebulaKeyUtilsV3::partTagPrefix(PartitionID partId) {
 }
 std::string NebulaKeyUtilsV3::getVertexKey(folly::StringPiece tagKey) {
   std::string key = tagKey.toString();
-  key[3] = static_cast<uint32_t>(kVertex);
+  key[0] = static_cast<uint32_t>(kVertex);
   key.resize(key.size() - sizeof(TagID));
   return key;
 }