diff --git a/src/hb-ft.cc b/src/hb-ft.cc
index 786b94b23..f5fd7cae0 100644
--- a/src/hb-ft.cc
+++ b/src/hb-ft.cc
@@ -213,9 +213,11 @@ _hb_ft_hb_font_check_changed (hb_font_t *font,
 {
   if (font->serial != ft_font->cached_serial)
   {
+    ft_font->lock.lock ();
     _hb_ft_hb_font_changed (font, ft_font->ft_face);
     ft_font->advance_cache.clear ();
     ft_font->cached_serial = font->serial;
+    ft_font->lock.unlock ();
     return true;
   }
   return false;