AutoMapper constructor

Not working

    public class Test
    {
        [Fact]
        public void MapChildViewModel()
        {
            Mapper.CreateMap<ViewModel, Dto>();

            var childViewModel = new ChildViewModel
            {
                ChildId = 5,
                ChildName = "Child"
            };

            var viewModel = new ViewModel
            {
                Name = "Parent",
                Id = 1,
                Child = childViewModel,
                Additional = "Foo"
            };

            Dto map = Mapper.Map<ViewModel, Dto>(viewModel);

            map.Id.Should().Be(1);
            map.Name.Should().Be(viewModel.Name);
            map.ChildName.Should().Be(childViewModel.ChildName);
        }
    }

    public class ViewModel
    {
        public string Name { get; set; }

        public int Id { get; set; }

        public string Additional { get; set; }

        public ChildViewModel Child { get; set; }
    }

    public class ChildViewModel
    {
        public int ChildId { get; set; }

        public string ChildName { get; set; }
    }

    public class Dto
    {
        public Dto(int id, string name, string childName)
        {
            this.Id = id;
            this.Name = name;
            this.ChildName = childName;
        }

        public int Id { get; private set; }

        public string Name { get; private set; }

        public string ChildName { get; private set; }
    }

Working

    public class Test
    {
        [Fact]
        public void MapChildViewModel()
        {
            Mapper.CreateMap<ViewModel, Dto>();

            var childViewModel = new ChildViewModel
            {
                Id = 5,
                Name = "Child"
            };

            var viewModel = new ViewModel
            {
                Name = "Parent",
                Id = 1,
                Child = childViewModel,
                Additional = "Foo"
            };

            Dto map = Mapper.Map<ViewModel, Dto>(viewModel);

            map.Id.Should().Be(1);
            map.Name.Should().Be(viewModel.Name);
            map.ChildName.Should().Be(childViewModel.Name);
        }
    }

    public class ViewModel
    {
        public string Name { get; set; }

        public int Id { get; set; }

        public string Additional { get; set; }

        public ChildViewModel Child { get; set; }
    }

    public class ChildViewModel
    {
        public int Id { get; set; }

        public string Name { get; set; }
    }

    public class Dto
    {
        public Dto(int id, string name, string childName)
        {
            this.Id = id;
            this.Name = name;
            this.ChildName = childName;
        }

        public int Id { get; private set; }

        public string Name { get; private set; }

        public string ChildName { get; private set; }
    }