how to Apply Master Pages By Folder in web.config

I have folder called Email which contains 20 aspx pages I am trying to apply master page to this email folder through web.config

i Got following error The relative virtual path 'MasterPage.Master' is not allowed here.

code in web.config

开发者_开发技巧

Please help in this issue

Use an application-relative path, like ~/Email/MasterPage.Master