diff --git a/src/hb-serialize.hh b/src/hb-serialize.hh
index 9fa0658b9..e7c6445d4 100644
--- a/src/hb-serialize.hh
+++ b/src/hb-serialize.hh
@@ -295,6 +295,7 @@ struct hb_serialize_context_t
       {
         const object_t::link_t &link = *link_it;
 	const object_t &child = *packed[link.objidx];
+	assert (link.bias <= parent.tail - parent.head);
 	unsigned offset = (child.head - parent.head) - link.bias;
 
 	if (link.is_wide)