Sharepoint 2013 Custom Master Page

Hi, 

I created a custom master page by copying the seattle.master and renamed to custom.master.

I then decided i don't want this custom master page and try to delete it but I can't delete it.

Sharepoint 2013 designer shows a error:

"Server error: This file may not be moved, deleted, renamed, or otherwise edited."

Does anyone know how to delete the custom.master file?

Thanks