AutoMapper mapping to a property of a nullable property

How can you map a property to a sub-property that may be null?

eg the following code will fail with a NullReferenceException because the Contact's User property is null.

using AutoMapper;

namespace AutoMapperTests
{
    class Program
    {
        static void Main( string[] args )
        {
            Mapper.CreateMap<Contact, ContactModel>()
                .ForMember( x => x.UserName,  opt => opt.MapFrom( y => y.User.UserName ) );

            Mapper.AssertConfigurationIsValid();

            var c = new Contact();

            var co = new ContactModel();

            Mapper.Map( c, co );
        }
    }

    public class User
    {
        public string UserName { get; set; }
    }

    public class Contact
    {
        public User User { get; set; }
    }

    public class ContactModel
    {
        public string UserName { get; set; }
    }
}

I'd like ContactModel's UserName to default to an empty string instead.

I have tried the NullSubstitute method, but I assume that's trying to operate with User.Username, rather than just on the User property.