diff --git a/src/emqx_types.erl b/src/emqx_types.erl
index 0c15bd288..3e53eafd1 100644
--- a/src/emqx_types.erl
+++ b/src/emqx_types.erl
@@ -95,7 +95,9 @@
              | ?MQTT_PROTO_V4
              | ?MQTT_PROTO_V5
              | non_neg_integer()
-             | binary()).
+             %% Some non-MQTT versions of protocol may be a binary type
+             | binary()
+             ).
 
 -type(qos() :: ?QOS_0 | ?QOS_1 | ?QOS_2).
 -type(qos_name() :: qos0 | at_most_once |