diff --git a/libraries/GCS_MAVLink/GCS_Common.cpp b/libraries/GCS_MAVLink/GCS_Common.cpp
index f17511b68c..87b1b55d34 100644
--- a/libraries/GCS_MAVLink/GCS_Common.cpp
+++ b/libraries/GCS_MAVLink/GCS_Common.cpp
@@ -1947,8 +1947,14 @@ void GCS_MAVLINK::handle_set_mode(const mavlink_message_t &msg)
 
     const MAV_RESULT result = _set_mode_common(_base_mode, _custom_mode);
 
-    // send ACK or NAK
-    mavlink_msg_command_ack_send(chan, MAVLINK_MSG_ID_SET_MODE, result);
+    // send ACK or NAK.  Note that this is extraodinarily improper -
+    // we are sending a command-ack for a message which is not a
+    // command.  The command we are acking (ID=11) doesn't actually
+    // exist, but if it did we'd probably be acking something
+    // completely unrelated to setting modes.
+    if (HAVE_PAYLOAD_SPACE(chan, MAVLINK_MSG_ID_COMMAND_ACK)) {
+        mavlink_msg_command_ack_send(chan, MAVLINK_MSG_ID_SET_MODE, result);
+    }
 }
 
 /*