User Is In Role "admin" But (Authorize (Roles="admin")) Won't Authenticate

I found a great answer on SO describing how to set up custom user roles, and I've done the same in my project. So in my Login service I have:

public ActionResult Login() {
// password authentication stuff omitted here
var roles = GetRoles(user.Type); // returns a string e.g. "admin,user"
var authTicket = new FormsAuthenticationTicket(
1,
userName,
DateTime.Now,
DateTime.Now.AddMinutes(20), // expiry
false,
roles,
"/");
var cookie = new HttpCookie(FormsAuthentication.FormsCookieName,
FormsAuthentication.Encrypt(authTicket));
Response.Cookies.Add(cookie);
return new XmlResult(xmlDoc); // don't worry so much about this - returns XML as ActionResult
}
And in Global.asax.cs, I have (copied verbatim from the other answer):
protected void Application_AuthenticateRequest(Object sender, EventArgs e) {
var authCookie = Context.Request.Cookies[FormsAuthentication.FormsCookieName];
if (authCookie != null) {
var authTicket = FormsAuthentication.Decrypt(authCookie.Value);
var roles = authTicket.UserData.Split(new Char[] { ',' });
var userPrincipal = new GenericPrincipal(new GenericIdentity(authTicket.Name), roles);
Context.User = userPrincipal;
}
}
Then, in my ServicesController class, I have:
[Authorize(Roles = "admin")]
//[Authorize]
public ActionResult DoAdminStuff() {
...
}

I login as a user with the "admin" role, and that works. Then I call /services/doadminstuff - and I get access denied, even though when I put a breakpoint in Global.asax.cs, I can see that my roles do include "admin". If I comment out the first Authorize attribute (with roles) and just use a plain vanilla Authorize, then I can access the service.