diff --git a/src/hb-ot-hdmx-table.hh b/src/hb-ot-hdmx-table.hh
index a506c2940..deedd3808 100644
--- a/src/hb-ot-hdmx-table.hh
+++ b/src/hb-ot-hdmx-table.hh
@@ -53,6 +53,15 @@ struct DeviceRecord
       return this->subset_plan->gids_to_retain_sorted.len;
     }
 
+    inline unsigned int get_size () const
+    {
+      unsigned int raw_size = min_size + len () * HBUINT8::static_size;
+      if (raw_size % 4)
+        /* Align to 32 bits */
+        return raw_size + (4 - (raw_size % 4));
+      return raw_size;
+    }
+
     inline const HBUINT8& operator [] (unsigned int i) const
     {
       if (unlikely (i >= len())) return Null(HBUINT8);
@@ -65,16 +74,14 @@ struct DeviceRecord
   {
     TRACE_SERIALIZE (this);
 
-    if (unlikely (!c->extend_min ((*this))))  return_trace (false);
+    if (unlikely (!c->allocate_size<DeviceRecord> (subset_view.get_size())))
+      return_trace (false);
+
     this->pixel_size.set (subset_view.source_device_record->pixel_size);
     this->max_width.set (subset_view.source_device_record->max_width);
 
     for (unsigned int i = 0; i < subset_view.len(); i++)
-    {
-      HBUINT8 *width = c->extend (this->widths[i]);
-      if (unlikely (!width)) return_trace (false);
-      width->set (subset_view[i]);
-    }
+      widths[i].set (subset_view[i]);
 
     return_trace (true);
   }