diff --git a/src/hb-ot-layout-gsubgpos-private.hh b/src/hb-ot-layout-gsubgpos-private.hh
index 661085d51..cbaa64880 100644
--- a/src/hb-ot-layout-gsubgpos-private.hh
+++ b/src/hb-ot-layout-gsubgpos-private.hh
@@ -388,7 +388,7 @@ struct hb_ot_apply_context_t :
     inline bool prev (void)
     {
       assert (num_items > 0);
-      while (idx >= num_items)
+      while (idx > num_items - 1)
       {
 	idx--;
 	const hb_glyph_info_t &info = c->buffer->out_info[idx];