AuthenticationManager.SignIn() is not signing in

I was missing the AuthenticationType = DefaultAuthenticationTypes.ApplicationCookie from the OWIN startup class:

var authenticationOptions = new CookieAuthenticationOptions
{
    LoginPath = new PathString("/Account/SignIn"),
    AuthenticationType = DefaultAuthenticationTypes.ApplicationCookie
};

appBuilder.UseCookieAuthentication(authenticationOptions);

It's a shame that there isn't a nice, useful error. I don't like programs that silently fail.