diff --git a/plugins/providers/virtualbox/config.rb b/plugins/providers/virtualbox/config.rb
index 0428a1136..c71c2f4d9 100644
--- a/plugins/providers/virtualbox/config.rb
+++ b/plugins/providers/virtualbox/config.rb
@@ -26,6 +26,13 @@ module VagrantPlugins
       def customize(command)
         @customizations << command
       end
+
+      # This is the hook that is called to finalize the object before it
+      # is put into use.
+      def finalize!
+        # Default is to not show a GUI
+        @gui = false if @gui == UNSET_VALUE
+      end
     end
   end
 end