diff --git a/src/hb-subset-plan.cc b/src/hb-subset-plan.cc
index fad75d0b3..2e7b71b8e 100644
--- a/src/hb-subset-plan.cc
+++ b/src/hb-subset-plan.cc
@@ -183,9 +183,11 @@ _create_old_gid_to_new_gid_map (const hb_face_t *face,
     | hb_sink (reverse_glyph_map)
     ;
 
-    // TODO(grieger): Should we discard glyphs past the max glyph to keep?
-    // *num_glyphs = + hb_iter (all_gids_to_retain) | hb_reduce (hb_max, 0);
-    *num_glyphs = face->get_num_glyphs ();
+    unsigned max_glyph =
+    + hb_iter (all_gids_to_retain)
+    | hb_reduce (hb_max, 0)
+    ;
+    *num_glyphs = max_glyph + 1;
   }
 
   + reverse_glyph_map->iter ()
diff --git a/test/api/fonts/Roboto-Regular.a.retaingids.ttf b/test/api/fonts/Roboto-Regular.a.retaingids.ttf
new file mode 100644
index 000000000..183b148ca
Binary files /dev/null and b/test/api/fonts/Roboto-Regular.a.retaingids.ttf differ
diff --git a/test/api/fonts/SourceHanSans-Regular.41,4C2E.retaingids.otf b/test/api/fonts/SourceHanSans-Regular.41,4C2E.retaingids.otf
index 906bdbefd..fa2a0e446 100644
Binary files a/test/api/fonts/SourceHanSans-Regular.41,4C2E.retaingids.otf and b/test/api/fonts/SourceHanSans-Regular.41,4C2E.retaingids.otf differ
diff --git a/test/api/test-subset-glyf.c b/test/api/test-subset-glyf.c
index 4671156f9..dd9833c58 100644
--- a/test/api/test-subset-glyf.c
+++ b/test/api/test-subset-glyf.c
@@ -305,6 +305,30 @@ test_subset_glyf_retain_gids (void)
   hb_face_destroy (face_ac);
 }
 
+static void
+test_subset_glyf_retain_gids_truncates (void)
+{
+  hb_face_t *face_abc = hb_test_open_font_file ("fonts/Roboto-Regular.abc.ttf");
+  hb_face_t *face_a = hb_test_open_font_file ("fonts/Roboto-Regular.a.retaingids.ttf");
+
+  hb_set_t *codepoints = hb_set_create();
+  hb_face_t *face_abc_subset;
+  hb_set_add (codepoints, 97);
+
+  hb_subset_input_t *input = hb_subset_test_create_input (codepoints);
+  hb_subset_input_set_retain_gids (input, true);
+  face_abc_subset = hb_subset_test_create_subset (face_abc, input);
+  hb_set_destroy (codepoints);
+
+  hb_subset_test_check (face_a, face_abc_subset, HB_TAG ('g','l','y','f'));
+  hb_subset_test_check (face_a, face_abc_subset, HB_TAG ('l','o','c', 'a'));
+  check_maxp_num_glyphs(face_abc_subset, 2, true);
+
+  hb_face_destroy (face_abc_subset);
+  hb_face_destroy (face_abc);
+  hb_face_destroy (face_a);
+}
+
 // TODO(grieger): test for long loca generation.
 
 int
@@ -322,6 +346,7 @@ main (int argc, char **argv)
   hb_test_add (test_subset_glyf_with_gsub);
   hb_test_add (test_subset_glyf_without_gsub);
   hb_test_add (test_subset_glyf_retain_gids);
+  hb_test_add (test_subset_glyf_retain_gids_truncates);
 
   return hb_test_run();
 }
diff --git a/test/subset/data/expected/basics/Roboto-Regular.abc.drop-hints-retain-gids.61.ttf b/test/subset/data/expected/basics/Roboto-Regular.abc.drop-hints-retain-gids.61.ttf
index 1a0d5bdd0..128eae016 100644
Binary files a/test/subset/data/expected/basics/Roboto-Regular.abc.drop-hints-retain-gids.61.ttf and b/test/subset/data/expected/basics/Roboto-Regular.abc.drop-hints-retain-gids.61.ttf differ
diff --git a/test/subset/data/expected/basics/Roboto-Regular.abc.drop-hints-retain-gids.62.ttf b/test/subset/data/expected/basics/Roboto-Regular.abc.drop-hints-retain-gids.62.ttf
index 257184b92..2d2b65b0d 100644
Binary files a/test/subset/data/expected/basics/Roboto-Regular.abc.drop-hints-retain-gids.62.ttf and b/test/subset/data/expected/basics/Roboto-Regular.abc.drop-hints-retain-gids.62.ttf differ
diff --git a/test/subset/data/expected/basics/Roboto-Regular.abc.retain-gids.61.ttf b/test/subset/data/expected/basics/Roboto-Regular.abc.retain-gids.61.ttf
index d3a67eafa..a699eea0b 100644
Binary files a/test/subset/data/expected/basics/Roboto-Regular.abc.retain-gids.61.ttf and b/test/subset/data/expected/basics/Roboto-Regular.abc.retain-gids.61.ttf differ
diff --git a/test/subset/data/expected/basics/Roboto-Regular.abc.retain-gids.62.ttf b/test/subset/data/expected/basics/Roboto-Regular.abc.retain-gids.62.ttf
index 4ff6e338c..eb84f9c9f 100644
Binary files a/test/subset/data/expected/basics/Roboto-Regular.abc.retain-gids.62.ttf and b/test/subset/data/expected/basics/Roboto-Regular.abc.retain-gids.62.ttf differ