diff --git a/src/emqttd_session.erl b/src/emqttd_session.erl
index 4cb927ef4..d2163d3f4 100644
--- a/src/emqttd_session.erl
+++ b/src/emqttd_session.erl
@@ -170,9 +170,9 @@ destroy(SessPid, ClientId) ->
 subscribe(SessPid, TopicTable) ->
     subscribe(SessPid, TopicTable, fun(_) -> ok end).
 
--spec subscribe(pid(), [{binary(), mqtt_qos()}], Callback :: fun()) -> ok.
-subscribe(SessPid, TopicTable, Callback) ->
-    gen_server2:cast(SessPid, {subscribe, TopicTable, Callback}).
+-spec subscribe(pid(), [{binary(), mqtt_qos()}], AckFun :: fun()) -> ok.
+subscribe(SessPid, TopicTable, AckFun) ->
+    gen_server2:cast(SessPid, {subscribe, TopicTable, AckFun}).
 
 %%------------------------------------------------------------------------------
 %% @doc Publish message
@@ -296,20 +296,20 @@ handle_call(Req, _From, State) ->
     lager:error("Unexpected Request: ~p", [Req]),
     {reply, ok, State}.
 
-handle_cast({subscribe, TopicTable0, Callback}, Session = #session{
+handle_cast({subscribe, TopicTable0, AckFun}, Session = #session{
                 client_id = ClientId, subscriptions = Subscriptions}) ->
 
     TopicTable = emqttd_broker:foldl_hooks('client.subscribe', [ClientId], TopicTable0),
 
     case TopicTable -- Subscriptions of
         [] ->
-            catch Callback([Qos || {_, Qos} <- TopicTable]),
+            catch AckFun([Qos || {_, Qos} <- TopicTable]),
             noreply(Session);
         _  ->
             %% subscribe first and don't care if the subscriptions have been existed
             {ok, GrantedQos} = emqttd_pubsub:subscribe(TopicTable),
 
-            catch Callback(GrantedQos),
+            catch AckFun(GrantedQos),
 
             emqttd_broker:foreach_hooks('client.subscribe.after', [ClientId, TopicTable]),