Rhino mock of inherited class

Im trying to mock System.Web.Mvc.ModelStateDictionary which does not have virtual methods. So I've tried to make a sub class in this way:

public abstract class ModelStateDictionaryMock : ModelStateDictionary
{
    public new abstract int Count { get; }

    public new abstract bool IsReadOnly { get; }

    public new abstract bool IsValid { get; }

    public new abstract ICollection<string> Keys { get; }

    public new abstract ICollection<ModelState> Values { get; }

    public new abstract ModelState this[string key] { get; set; }

    public new abstract void Add(KeyValuePair<string, ModelState> item);

    public new abstract void Add(string key, ModelState value);

    public new abstract void AddModelError(string key, Exception exception);

    public new abstract void AddModelError(string key, string errorMessage);

    public new abstract void Clear();

    public new abstract bool Contains(KeyValuePair<string, ModelState> item);

    public new abstract bool ContainsKey(string key);

    public new abstract void CopyTo(KeyValuePair<string, ModelState>[] array, int arrayIndex);

    public new abstract IEnumerator<KeyValuePair<string, ModelState>> GetEnumerator();

    public new abstract bool IsValidField(string key);

    public new abstract void Merge(ModelStateDictionary dictionary);

    public new abstract bool Remove(KeyValuePair<string, ModelState> item);

    public new abstract bool Remove(string key);

    public new abstract void SetModelValue(string key, ValueProviderResult value);

    public new abstract bool TryGetValue(string key, out ModelState value);
}

However when I generate a mock using var modelStateDictionary = MockRepository.GenerateMock<ModelStateDictionaryMock>(); I get an object which has a double set of properties, like this:

Doing modelStateDictionary.Count accesses the base property, so my mock does not work. Does anyone know how to get around this problem?