diff --git a/src/hb-subset-plan.cc b/src/hb-subset-plan.cc
index 6b33c17b6..e244da199 100644
--- a/src/hb-subset-plan.cc
+++ b/src/hb-subset-plan.cc
@@ -179,7 +179,7 @@ _create_old_gid_to_new_gid_map (const hb_face_t *face,
 
     unsigned max_glyph =
     + hb_iter (all_gids_to_retain)
-    | hb_reduce (hb_max, 0)
+    | hb_reduce (hb_max, 0u)
     ;
     *num_glyphs = max_glyph + 1;
   }