diff --git a/apps/emqtt/include/emqtt_packet.hrl b/apps/emqtt/include/emqtt_packet.hrl
index 36442a923..105a939cf 100644
--- a/apps/emqtt/include/emqtt_packet.hrl
+++ b/apps/emqtt/include/emqtt_packet.hrl
@@ -98,10 +98,11 @@
 %%------------------------------------------------------------------------------
 %% MQTT Packets
 %%------------------------------------------------------------------------------
+-type mqtt_clientid()  :: binary().
 -type mqtt_packet_id() :: 1..16#ffff | undefined.
 
 -record(mqtt_packet_connect,  {
-    clientid    = <<>>              :: binary(),
+    clientid    = <<>>              :: mqtt_clientid(),
     proto_ver   = ?MQTT_PROTO_V311  :: mqtt_vsn(),
     proto_name  = <<"MQTT">>        :: binary(),
     will_retain = false             :: boolean(),