AutoMapper ProjectUsing not being called

I'm trying to use the ProjectUsing feature of AutoMapper to only select the columns I need through a LINQ expression but it seems the expression itself does not get called at run-time or via unit testing.

As a test I am just putting a fixed value into the AlternateId property but the assert below always fails. This also fails with single instances (not in a queryable list) and at run-time via Entity Framework 6.

    class MapFrom
    {
        public int Id { get; set; }
    }
    class MapTo
    {
        public int AlternateId { get; set; }
    }

    [TestMethod]
    public void Automapper_projectusing_test()
    {
        AutoMapper
            .Mapper
            .CreateMap<MapFrom, MapTo>()
            .ProjectUsing(src => new MapTo { AlternateId = 88 });
        var products = new List<MapFrom>();
        products.Add(new MapFrom());

        var mapped = products
            .AsQueryable()    // Just in case ProjectUsing only works with IQueryable.
            .Project()
            .To<MapTo>()
            .ToList();

        Assert.AreEqual(88, mapped.Single().AlternateId); // Fails, AlternateId equals 0.
    }

Using AutoMapper v3.3.1, NCrunch confirms that the mapping expression code is never executed.

Why is AutoMapper not executing this expression, perhaps I'm missing a key step?