diff --git a/src/hb-serialize.hh b/src/hb-serialize.hh
index 4e270ce8c..27c8c1778 100644
--- a/src/hb-serialize.hh
+++ b/src/hb-serialize.hh
@@ -292,7 +292,7 @@ struct hb_serialize_context_t
       for (const object_t::link_t &link : parent->links)
       {
 	const object_t &child = *packed[link.objidx];
-	assert (link.bias <= parent->tail - parent->head);
+	assert (link.bias <= (size_t) (parent->tail - parent->head));
 	unsigned offset = (child.head - parent->head) - link.bias;
 
 	if (link.is_wide)