Using [Authorize(Roles= with an AD security group

Hi,

I'm putting in an authentication mechanism for my web app. If I use a specific user account then it's fine, however, if I try and use an AD security group then it prompts for authentication despite being in the user group.

Works:

[Authorize(Roles = "DOMAIN\\user.name")]

Does not work:

[Authorize(Roles = "DOMAIN\\Security Group")]

Using MVC 5

Any pointers?