diff --git a/hbjs.cc b/hbjs.cc
index 67ad266..8a3255a 100644
--- a/hbjs.cc
+++ b/hbjs.cc
@@ -170,7 +170,7 @@ static hb_bool_t do_trace (hb_buffer_t *buffer,
   if (user_data->stop_phase != HB_SHAPE_DONT_STOP) {
     // Do we need to start stopping?
     char buf[12];
-    snprintf (buf, 12, "%d ", user_data->stop_at);
+    snprintf (buf, 12, "%u ", user_data->stop_at);
     if ((user_data->current_phase == user_data->stop_phase) &&
         (strncmp(message, "end lookup ", 11) == 0) &&
         (strncmp(message + 11, buf, strlen(buf)) == 0)) {