diff --git a/test/api/hb-subset-test.h b/test/api/hb-subset-test.h
index efae17976..de7f274f2 100644
--- a/test/api/hb-subset-test.h
+++ b/test/api/hb-subset-test.h
@@ -90,16 +90,14 @@ hb_subset_test_open_font (const char *font_path)
 
 static inline hb_face_t *
 hb_subset_test_create_subset (hb_face_t *source,
-                              hb_set_t  *codepoints)
+                              const hb_set_t  *codepoints)
 {
   hb_subset_profile_t *profile = hb_subset_profile_create();
   hb_subset_input_t *input = hb_subset_input_create_or_fail ();
 
   hb_set_t * input_codepoints = hb_subset_input_unicode_set (input);
-  hb_codepoint_t codepoint = -1;
-  while (hb_set_next (codepoints, &codepoint)) {
-    hb_set_add (input_codepoints, codepoint);
-  }
+
+  hb_set_union (input_codepoints, codepoints);
 
   hb_face_t *subset = hb_subset (source, profile, input);
   g_assert (subset);