diff --git a/src/emqx_log_action.erl b/src/emqx_log_action.erl
index 5ef6659..ef6af68 100644
--- a/src/emqx_log_action.erl
+++ b/src/emqx_log_action.erl
@@ -34,10 +34,16 @@ kvs_to_iolist(KVs) ->
         ", ",
         lists:map(
             fun({K, V}) -> [to_iolist(K), ": ", to_iolist(V)] end,
-            KVs
+            filter_undefined(KVs)
         )
     ).
 
+filter_undefined(KVs) ->
+    lists:filter(
+        fun({_, V}) -> V =/= undefined end,
+        KVs
+    ).
+
 try_format_unicode(undefined) ->
     "undefined";
 try_format_unicode(Char) ->