diff --git a/server/imserver/dbengine.cc b/server/imserver/dbengine.cc
index 85d95ff..26a5ae8 100644
--- a/server/imserver/dbengine.cc
+++ b/server/imserver/dbengine.cc
@@ -179,8 +179,8 @@ void DBEngine::RefreshIdx()
         DBEngine::Instance()->ExecAsyncQuery
             (
              conn_info,
-             "SELECT MAX(idx) FROM friend_apply UNION "
-             "SELECT MAX(idx) FROM event;",
+             "SELECT IFNULL(MAX(idx), 0) FROM friend_apply UNION ALL "
+             "SELECT IFNULL(MAX(idx), 0) FROM event;",
              {
              },
              a8::XParams()