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
Hmm, after deleting it and typing it in manually, started working. Bizarre.