diff --git a/include/emqx.hrl b/include/emqx.hrl
index d676be9cf..ee048f1fa 100644
--- a/include/emqx.hrl
+++ b/include/emqx.hrl
@@ -64,7 +64,7 @@
           %% Message flags
           flags :: #{atom() => boolean()},
           %% Message headers, or MQTT 5.0 Properties
-          headers = #{},
+          headers :: map(),
           %% Topic that the message is published to
           topic :: binary(),
           %% Message Payload