Calling base CreateUser method from extended SqlMembershipProvider

Hi,
I'm getting a null reference exception (Object reference not set to an instance of an object.) when I call base.CreateUser from my provider class which inherits from SqlMembershipProvider:
    public class InheritingMembershipProvider : SqlMembershipProvider
    {
        public InheritingMembershipUser CreateUser(string username, string password, string email,
            string passwordQuestion, string passwordAnswer, bool isApproved,
            object providerUserKey, string FirstName, string LastName, DateTime DateOfBirth,
            out MembershipCreateStatus status)
        {
            // Call the SqlMe ...