I'm trying to create a new, virtual role that checks for a particular key to be present within a user's session, but the session object is always null. Here's a simplified version of the virtual role. What am I doing wrong?

public class CustomSessionRole : EPiServer.Security.VirtualRoleProviderBase
{
    public override bool IsInVirtualRole(IPrincipal principal, object context)
    {
        // HttpContext.Current.Session is always null
        if (HttpContext.Current == null || HttpContext.Current.Session == null) return false;

        // Never fires
        return HttpContext.Current.Session["someKey"] != null;
    }
}

The virtual role is being added in the web.config's <virtualRoles> section as follows:

<add name="CustomSession" type="MyNamespace.CustomSessionRole, MyDLL" mode="Any" />