diff --git a/apps/emqx/src/emqx_map_lib.erl b/apps/emqx/src/emqx_map_lib.erl
index cdc167729..8a5ecefad 100644
--- a/apps/emqx/src/emqx_map_lib.erl
+++ b/apps/emqx/src/emqx_map_lib.erl
@@ -31,7 +31,7 @@
         ]).
 
 -export_type([config_key/0, config_key_path/0]).
--type config_key() :: atom() | binary() | string().
+-type config_key() :: atom() | binary() | [byte()].
 -type config_key_path() :: [config_key()].
 -type convert_fun() :: fun((...) -> {K1::any(), V1::any()} | drop).