diff --git a/src/hb-open-file.hh b/src/hb-open-file.hh
index 317c49bbb..6eee5827c 100644
--- a/src/hb-open-file.hh
+++ b/src/hb-open-file.hh
@@ -119,7 +119,7 @@ typedef struct OpenTypeOffsetTable
   public:
 
   template <typename Iterator,
-	    hb_requires (hb_is_iterator (Iterator))>
+	    hb_requires ((hb_is_source_of<Iterator, hb_pair_t<hb_tag_t, hb_blob_t *>>::value))>
   bool serialize (hb_serialize_context_t *c,
 		  hb_tag_t sfnt_tag,
 		  Iterator it)
@@ -493,7 +493,7 @@ struct OpenTypeFontFile
   }
 
   template <typename Iterator,
-	    hb_requires (hb_is_iterator (Iterator))>
+	    hb_requires ((hb_is_source_of<Iterator, hb_pair_t<hb_tag_t, hb_blob_t *>>::value))>
   bool serialize_single (hb_serialize_context_t *c,
 			 hb_tag_t sfnt_tag,
 			 Iterator items)