diff --git a/apps/emqx_bridge_mqtt/src/emqx_bridge_mqtt_connector.erl b/apps/emqx_bridge_mqtt/src/emqx_bridge_mqtt_connector.erl
index 92491991f..d507d11b8 100644
--- a/apps/emqx_bridge_mqtt/src/emqx_bridge_mqtt_connector.erl
+++ b/apps/emqx_bridge_mqtt/src/emqx_bridge_mqtt_connector.erl
@@ -57,7 +57,9 @@
 -define(HEALTH_CHECK_TIMEOUT, 1000).
 -define(INGRESS, "I").
 -define(EGRESS, "E").
--define(IS_NO_PREFIX(P), (P =:= undefined orelse P =:= <<>>)).
+-define(NO_PREFIX, <<>>).
+-define(IS_NO_PREFIX(P), (P =:= undefined orelse P =:= ?NO_PREFIX)).
+-define(MAX_PREFIX_BYTES, 19).
 
 %% ===================================================================
 %% When use this bridge as a data source, ?MODULE:on_message_received will be called
@@ -444,7 +446,7 @@ clientid(Name, _Conf = #{clientid_prefix := Prefix}) when
 ->
     {Prefix, emqx_bridge_mqtt_lib:clientid_base(Name)};
 clientid(Name, _Conf) ->
-    {undefined, emqx_bridge_mqtt_lib:clientid_base(Name)}.
+    {?NO_PREFIX, emqx_bridge_mqtt_lib:clientid_base(Name)}.
 
 %% @doc Start an ingress bridge worker.
 -spec connect([option() | {ecpool_worker_id, pos_integer()}]) ->
@@ -485,10 +487,9 @@ mk_client_opts(
 mk_clientid(WorkerId, {Prefix, ClientId}) when ?IS_NO_PREFIX(Prefix) ->
     %% When there is no prefix, try to keep the client ID length within 23 bytes
     emqx_bridge_mqtt_lib:bytes23(ClientId, WorkerId);
-mk_clientid(WorkerId, {Prefix, ClientId}) when size(Prefix) < 20 ->
-    %% Try to respect client ID prefix when it's less than 20 bytes
-    %% meaning there is at least 3 bytes to randomize
-    %% Must add $: for backward compatibility
+mk_clientid(WorkerId, {Prefix, ClientId}) when size(Prefix) =< ?MAX_PREFIX_BYTES ->
+    %% Try to respect client ID prefix when it's no more than 19 bytes,
+    %% meaning there are at least 4 bytes as hash space.
     emqx_bridge_mqtt_lib:bytes23_with_prefix(Prefix, ClientId, WorkerId);
 mk_clientid(WorkerId, {Prefix, ClientId}) ->
     %% There is no other option but to use a long client ID