Unable to delete Master Pages or Page Layouts?

I have just been tidying up my masterpage library in a WCM sitecollection and came across an issue when trying to delete some master pages.

When trying to delete I got the following error:





I know that this is not the case - no other pages use this master page!

Fortunatley there is a workaround for this. Simply create another folder in the masterpage folder and then drag the master page you want to delete into it. Then delete the folder and your master page is gone.