diff --git a/doc/configuration.txt b/doc/configuration.txt
index 4a27f8ab5..e75d84b17 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -16976,6 +16976,8 @@ mqtt_is_valid
   CONNECT and CONNACK packet types. CONNECT is the first message sent by the
   client and CONNACK is the first response sent by the server.
 
+  Only MQTT 3.1, 3.1.1 and 5.0 are supported.
+
   Example:
 
       acl data_in_buffer req.len ge 4