--- src/hb-ot-layout-gsubgpos.hh
+++ src/hb-ot-layout-gsubgpos.hh
@@ -387,7 +387,7 @@
 	     skip == matcher_t::SKIP_NO))
 	{
 	  num_items--;
-	  match_glyph_data++;
+	  if (match_glyph_data != nullptr) match_glyph_data++;
 	  return true;
 	}
 
@@ -414,7 +414,7 @@
 	     skip == matcher_t::SKIP_NO))
 	{
 	  num_items--;
-	  match_glyph_data++;
+	  if (match_glyph_data != nullptr) match_glyph_data++;
 	  return true;
 	}