Overriding AuthorizeCore in custom Authorize attribute results in "no suitable method found to override" error

I'm trying to build custom AuthorizeAttribute, so in my Core project (a class library) I have this code:

using System;
using System.Web;
using System.Web.Mvc;
using IVC.Core.Web;
using System.Linq;

namespace IVC.Core.Attributes
{
    public class TimeShareAuthorizeAttribute : AuthorizeAttribute
    {
        protected override bool AuthorizeCore(HttpContextBase httpContext)
        {
            if(!httpContext.Request.IsAuthenticated)
                return false;

            var rolesProvider = System.Web.Security.Roles.Providers["TimeShareRoleProvider"];

            string[] roles = rolesProvider.GetRolesForUser(httpContext.User.Identity.Name);

            if(roles.Contains(Website.Roles.RegisteredClient, StringComparer.OrdinalIgnoreCase))
            {
                return true;
            }

            return false;
        }

        protected override void HandleUnauthorizedRequest(AuthorizationContext filterContext)
        {
            filterContext.Result = new RedirectResult("/TimeShare/Account/LogOn");

            base.HandleUnauthorizedRequest(filterContext);
        }
    }
}

When I try to build the thing I get this error: Error 2 'IVC.Core.Attributes.TimeShareAuthorizeAttribute.AuthorizeCore(System.Web.HttpContextBase)': no suitable method found to override ...

Am I missing something here? I've searched all over but every site I can find just tells me to do exactly what I did here. I'm using mvc2 btw.

• Edited to add: if I move the class to the mvc project in the same solution there's no compiler error.