diff --git a/util/options.cc b/util/options.cc
index 29e0b4fbf..349c19902 100644
--- a/util/options.cc
+++ b/util/options.cc
@@ -650,12 +650,12 @@ format_options_t::serialize (hb_buffer_t *buffer,
       if (!FT_Get_Glyph_Name (ft_face, info->codepoint, glyph_name, sizeof (glyph_name)))
 	g_string_append_printf (gs, "%s", glyph_name);
       else
-	g_string_append_printf (gs, "gid%d", info->codepoint);
+	g_string_append_printf (gs, "gid%u", info->codepoint);
     } else
-      g_string_append_printf (gs, "%d", info->codepoint);
+      g_string_append_printf (gs, "%u", info->codepoint);
 
     if (show_clusters)
-      g_string_append_printf (gs, "=%d", info->cluster);
+      g_string_append_printf (gs, "=%u", info->cluster);
 
     if (show_positions && (pos->x_offset || pos->y_offset)) {
       g_string_append_c (gs, '@');