diff --git a/src/emqttd_ws_client.erl b/src/emqttd_ws_client.erl
index f9cce71d0..1fc9fa60f 100644
--- a/src/emqttd_ws_client.erl
+++ b/src/emqttd_ws_client.erl
@@ -69,7 +69,7 @@ init([MqttEnv, WsPid, Req, ReplyChannel]) ->
     %% SendFun = fun(Payload) -> ReplyChannel({binary, Payload}) end,
     SendFun = fun(Packet) ->
                   Data = emqttd_serializer:serialize(Packet),
-                  emqttd_metrics:inc('bytes/sent', size(Data)),
+                  emqttd_metrics:inc('bytes/sent', iolist_size(Data)),
                   ReplyChannel({binary, Data})
               end,
     ProtoState = emqttd_protocol:init(Peername, SendFun,