diff --git a/src/hb-buffer-private.hh b/src/hb-buffer-private.hh
index 9bd80e0..e06a7fa 100644
--- a/src/hb-buffer-private.hh
+++ b/src/hb-buffer-private.hh
@@ -148,7 +148,7 @@
   HB_INTERNAL bool enlarge (unsigned int size);
 
   inline bool ensure (unsigned int size)
-  { return likely (size <= allocated) ? true : enlarge (size); }
+  { return likely (size < allocated) ? true : enlarge (size); }
 
   HB_INTERNAL bool make_room_for (unsigned int num_in, unsigned int num_out);