diff --git a/apps/emqx_utils/src/emqx_utils_maps.erl b/apps/emqx_utils/src/emqx_utils_maps.erl
index d64075511..6bec32ae3 100644
--- a/apps/emqx_utils/src/emqx_utils_maps.erl
+++ b/apps/emqx_utils/src/emqx_utils_maps.erl
@@ -210,6 +210,7 @@ binary_string_kv(K, V, JsonableFun) ->
         {K1, V1} -> {binary_string(K1), V1}
     end.
 
+%% [FIXME] this doesn't belong here
 binary_string([]) ->
     [];
 binary_string(Val) when is_list(Val) ->