diff --git a/apps/emqx_coap/src/emqx_coap_mqtt_adapter.erl b/apps/emqx_coap/src/emqx_coap_mqtt_adapter.erl
index f7f0b4eda..a6ea09881 100644
--- a/apps/emqx_coap/src/emqx_coap_mqtt_adapter.erl
+++ b/apps/emqx_coap/src/emqx_coap_mqtt_adapter.erl
@@ -58,6 +58,8 @@
 
 -define(SUBOPTS, #{rh => 0, rap => 0, nl => 0, qos => ?QOS_0, is_new => false}).
 
+-define(PROTO_VER, 1).
+
 %%--------------------------------------------------------------------
 %% API
 %%--------------------------------------------------------------------
@@ -260,7 +262,7 @@ packet_to_message(Topic, Payload,
                 emqx_message:make(ClientId, ?QOS_0, Topic, Payload)
                ),
     emqx_message:set_headers(
-      #{ proto_ver => 1
+      #{ proto_ver => ?PROTO_VER
        , protocol => coap
        , username => Username
        , peerhost => PeerHost}, Message).
@@ -335,7 +337,7 @@ conninfo(#state{peername = Peername,
       peercert => nossl,        %% TODO: dtls
       conn_mod => ?MODULE,
       proto_name => <<"CoAP">>,
-      proto_ver => 1,
+      proto_ver => ?PROTO_VER,
       clean_start => true,
       clientid => ClientId,
       username => undefined,
diff --git a/apps/emqx_exhook/src/emqx_exhook_mngr.erl b/apps/emqx_exhook/src/emqx_exhook_mngr.erl
index 0f3c8b8ab..d4c493cb8 100644
--- a/apps/emqx_exhook/src/emqx_exhook_mngr.erl
+++ b/apps/emqx_exhook/src/emqx_exhook_mngr.erl
@@ -295,7 +295,9 @@ put_pool_size(Val) ->
     persistent_term:put({?APP, pool_size}, Val).
 
 get_pool_size() ->
-    persistent_term:get({?APP, pool_size}).
+    %% Avoid the scenario that the parameter is not set after
+    %% the hot upgrade completed.
+    persistent_term:get({?APP, pool_size}, erlang:system_info(schedulers)).
 
 save(Name, ServerState) ->
     Saved = persistent_term:get(?APP, []),
diff --git a/apps/emqx_exhook/test/emqx_exhook_demo_svr.erl b/apps/emqx_exhook/test/emqx_exhook_demo_svr.erl
index 1d3896e14..b05748856 100644
--- a/apps/emqx_exhook/test/emqx_exhook_demo_svr.erl
+++ b/apps/emqx_exhook/test/emqx_exhook_demo_svr.erl
@@ -295,7 +295,7 @@ on_session_terminated(Req, Md) ->
      | {error, grpc_cowboy_h:error_response()}.
 on_message_publish(#{message := #{from := From} = Msg} = Req, Md) ->
     ?MODULE:in({?FUNCTION_NAME, Req}),
-    %io:format(standard_error, "fun: ~p, req: ~0p~n", [?FUNCTION_NAME, Req]),
+    %io:format("fun: ~p, req: ~0p~n", [?FUNCTION_NAME, Req]),
     %% some cases for testing
     case From of
         <<"baduser">> ->