diff --git a/apisix/core/config_etcd.lua b/apisix/core/config_etcd.lua
index e59dc6b32d71..169bf1e4dc63 100644
--- a/apisix/core/config_etcd.lua
+++ b/apisix/core/config_etcd.lua
@@ -141,7 +141,7 @@ local function do_run_watch(premature)
 
         watch_ctx.cli, err = get_etcd()
         if not watch_ctx.cli then
-            error("failed to create etcd instance: " .. string(err))
+            error("failed to create etcd instance: " .. tostring(err))
         end
 
         local rev = 0