--- a/gfx/harfbuzz/src/hb-ot-layout-gsubgpos-private.hh
+++ b/gfx/harfbuzz/src/hb-ot-layout-gsubgpos-private.hh
@@ -225,17 +225,17 @@ HB_BEGIN_DECLS
 
 static inline bool apply_lookup (hb_apply_context_t *c,
 				 unsigned int count, /* Including the first glyph */
 				 unsigned int lookupCount,
 				 const LookupRecord lookupRecord[], /* Array of LookupRecords--in design order */
 				 apply_lookup_func_t apply_func)
 {
   unsigned int end = MIN (c->buffer->len, c->buffer->i + c->context_length);
-  if (unlikely (c->buffer->i + count > end))
+  if (unlikely (count == 0 || c->buffer->i + count > end))
     return false;
 
   /* TODO We don't support lookupRecord arrays that are not increasing:
    *      Should be easy for in_place ones at least. */
 
   /* Note: If sublookup is reverse, i will underflow after the first loop
    * and we jump out of it.  Not entirely disastrous.  So we don't check
    * for reverse lookup here.