diff --git a/libraries/GCS_MAVLink/GCS.h b/libraries/GCS_MAVLink/GCS.h
index 5cede26e55..f412d3c8cc 100644
--- a/libraries/GCS_MAVLink/GCS.h
+++ b/libraries/GCS_MAVLink/GCS.h
@@ -51,6 +51,7 @@ enum ap_message {
     MSG_BATTERY2,
     MSG_CAMERA_FEEDBACK,
     MSG_MOUNT_STATUS,
+    MSG_OPTICAL_FLOW,
     MSG_RETRY_DEFERRED // this must be last
 };