diff --git a/apisix/discovery/dns/init.lua b/apisix/discovery/dns/init.lua
index 601de0ebc9ee..197b7580d996 100644
--- a/apisix/discovery/dns/init.lua
+++ b/apisix/discovery/dns/init.lua
@@ -78,7 +78,7 @@ function _M.init_worker()
 
     local client, err = core.dns_client.new(opts)
     if not client then
-        error("failed to init the dns client: ", err)
+        error("failed to init the dns client: " .. (err or "unknown"))
         return
     end