diff --git a/monitor.c b/monitor.c
index 9cf419bb1d..c6eac608a3 100644
--- a/monitor.c
+++ b/monitor.c
@@ -270,6 +270,7 @@ static void monitor_puts(Monitor *mon, const char *str)
     char c;
 
     for(;;) {
+        assert(mon->outbuf_index < sizeof(mon->outbuf) - 1);
         c = *str++;
         if (c == '\0')
             break;