diff --git a/monitor.c b/monitor.c
index ae125b8..f94794d 100644
--- a/monitor.c
+++ b/monitor.c
@@ -2258,6 +2258,7 @@ static int do_balloon(Monitor *mon, const QDict *params,
         return -1;
     }
 
+    cb(opaque, NULL);
     return 0;
 }