diff --git a/src/hb-ot-layout-gsub-table.hh b/src/hb-ot-layout-gsub-table.hh
index a81cd79c2..66fcb3f3a 100644
--- a/src/hb-ot-layout-gsub-table.hh
+++ b/src/hb-ot-layout-gsub-table.hh
@@ -1321,28 +1321,10 @@ GSUB::substitute_start (hb_font_t *font, hb_buffer_t *buffer)
 
   const GDEF &gdef = *hb_ot_layout_from_face (font->face)->gdef;
   unsigned int count = buffer->len;
-  hb_glyph_info_t *info = buffer->info;
   for (unsigned int i = 0; i < count; i++)
   {
-    unsigned int props = gdef.get_glyph_props (info[i].codepoint);
-    if (!props)
-    {
-      /* Never mark default-ignorables as marks.
-       * They won't get in the way of lookups anyway,
-       * but having them as mark will cause them to be skipped
-       * over if the lookup-flag says so, but at least for the
-       * Mongolian variation selectors, looks like Uniscribe
-       * marks them as non-mark.  Some Mongolian fonts without
-       * GDEF rely on this.  Another notable character that
-       * this applies to is COMBINING GRAPHEME JOINER. */
-      props = (_hb_glyph_info_get_general_category (&info[i]) !=
-	       HB_UNICODE_GENERAL_CATEGORY_NON_SPACING_MARK ||
-	       _hb_glyph_info_is_default_ignorable (&info[i])) ?
-	      HB_OT_LAYOUT_GLYPH_PROPS_BASE_GLYPH :
-	      HB_OT_LAYOUT_GLYPH_PROPS_MARK;
-    }
-    _hb_glyph_info_set_glyph_props (&info[i], props);
-    _hb_glyph_info_clear_lig_props (&info[i]);
+    _hb_glyph_info_set_glyph_props (&buffer->info[i], gdef.get_glyph_props (buffer->info[i].codepoint));
+    _hb_glyph_info_clear_lig_props (&buffer->info[i]);
     buffer->info[i].syllable() = 0;
   }
 }
diff --git a/src/hb-ot-shape.cc b/src/hb-ot-shape.cc
index 987279878..cf916cc82 100644
--- a/src/hb-ot-shape.cc
+++ b/src/hb-ot-shape.cc
@@ -529,6 +529,32 @@ hb_ot_map_glyphs_fast (hb_buffer_t  *buffer)
   buffer->content_type = HB_BUFFER_CONTENT_TYPE_GLYPHS;
 }
 
+static inline void
+hb_synthesize_glyph_classes (hb_ot_shape_context_t *c)
+{
+  unsigned int count = c->buffer->len;
+  hb_glyph_info_t *info = c->buffer->info;
+  for (unsigned int i = 0; i < count; i++)
+  {
+    hb_ot_layout_glyph_props_flags_t klass;
+
+    /* Never mark default-ignorables as marks.
+     * They won't get in the way of lookups anyway,
+     * but having them as mark will cause them to be skipped
+     * over if the lookup-flag says so, but at least for the
+     * Mongolian variation selectors, looks like Uniscribe
+     * marks them as non-mark.  Some Mongolian fonts without
+     * GDEF rely on this.  Another notable character that
+     * this applies to is COMBINING GRAPHEME JOINER. */
+    klass = (_hb_glyph_info_get_general_category (&info[i]) !=
+	     HB_UNICODE_GENERAL_CATEGORY_NON_SPACING_MARK ||
+	     _hb_glyph_info_is_default_ignorable (&info[i])) ?
+	    HB_OT_LAYOUT_GLYPH_PROPS_BASE_GLYPH :
+	    HB_OT_LAYOUT_GLYPH_PROPS_MARK;
+    _hb_glyph_info_set_glyph_props (&info[i], klass);
+  }
+}
+
 static inline void
 hb_ot_substitute_default (hb_ot_shape_context_t *c)
 {
@@ -560,6 +586,9 @@ hb_ot_substitute_complex (hb_ot_shape_context_t *c)
 
   hb_ot_layout_substitute_start (c->font, buffer);
 
+  if (!hb_ot_layout_has_glyph_classes (c->face))
+    hb_synthesize_glyph_classes (c);
+
   c->plan->substitute (c->font, buffer);
 
   return;