Mock Generic Repository with contexts

I have a generic repository for several contexts. I can extend it and use it for services without problem. However, When I mock it, this.Entities in the constructor is always null.

GenericDataRepsository

public class GenericDataRepository<T, C> : IGenericDataRepository<T, C> where T : class where C : DbContext, new() {

    protected C context;
    protected IDbSet<T> Entities;

    public GenericDataRepository() {
        this.context = new C();
        this.Entities = context.Set<T>();
    }

    public GenericDataRepository(C context) {
        this.context = context;
        this.Entities = context.Set<T>();  // Problem is here. Entities didn't get set when mocking
    }

    public virtual IEnumerable<T> FindBy(Expression<Func<T, bool>> predicate) {
        return Entities.Where(predicate).ToList();
    }

    // omitted Add, Update, Delte
}

Unit Test

 [Test]
    public void GetBIlloTo() {
        var data = new List<BillTo> {
            new BillTo { CONTACTID = 12, COUNTRY = "USA" }
        }.AsQueryable();

        var mockSet = Substitute.For<DbSet<BillTo>, IQueryable<BillTo>>();

        ((IQueryable<BillTo>)mockSet).Provider.Returns(data.Provider);
        ((IQueryable<BillTo>)mockSet).Expression.Returns(data.Expression);
        ((IQueryable<BillTo>)mockSet).ElementType.Returns(data.ElementType);
((IQueryable<BillTo>)mockSet).GetEnumerator().Returns(data.GetEnumerator());

        var mockContext = Substitute.For<MyEntities>();
        mockContext.BillToes.Returns(mockSet);
        var repo = new GenericDataRepository<BillTo, MyEntities>(mockContext);

        //// Act
        var actual = repo.FindBy(r => r.CONTACTID == 12);

        //// Assert
        Assert.AreEqual(actual.Count(), 1);
    }

MyEntities - generated by EF and I didn't change it.

 public partial class MyEntities : DbContext
{
    public MyEntities()
        : base("name=MyEntities")
    {
    }

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        throw new UnintentionalCodeFirstException();
    }

    public virtual DbSet<BillTo> BillToes { get; set; }
}

I'm using EF database first, NUnit, and Nsubstitute. Tried Moq as well, same problem. So, Why didn't Entities get set?