Automapper - v5.2.0 Nullsubstitute String working with Map but not with ProjectTo

Question

I tried to use AutoMapper NullSubstitute feature with source member and destination member of type string.

It doesn't seem to work with Projection.

As an example, I adapted src/UnitTests/Projection/NullSubstitutes.cs.

namespace AutoMapper.UnitTests.Projection
{
    using System.Collections.Generic;
    using System.Linq;
    using QueryableExtensions;

    using Xunit;

    public class NullSubstitutesWithMapFrom
    {
        private List<Dest> _dests;

        public class Source
        {
            public string Value { get; set; }
        }

        public class Dest
        {
            public string ValuePropertyNotMatching { get; set; }
        }

        [Fact]
        public void Can_substitute_null_values()
        {
            MapperConfiguration Configuration = new MapperConfiguration(cfg =>
          {
              cfg.CreateMap<Source, Dest>().ForMember(m => m.ValuePropertyNotMatching, opt =>
              {
                  opt.MapFrom(src => src.Value);
                  opt.NullSubstitute("5");
              });

          });

            var source = new[] { new Source { Value = null } }.AsQueryable();
            _dests = source.ProjectTo<Dest>(Configuration).ToList();

            Assert.Equal("5", _dests[0].ValuePropertyNotMatching);
        }
    }
}

Expected : "5" equals "5" Actual : "5" equals null

With Map everything is ok

namespace AutoMapper.UnitTests.Projection
{
    using System.Collections.Generic;
    using System.Linq;

    using Xunit;

    public class NullSubstitutesWithMapFrom
    {
        private List<Dest> _dests;

        public class Source
        {
            public string Value { get; set; }
        }

        public class Dest
        {
            public string ValuePropertyNotMatching { get; set; }
        }

        [Fact]
        public void Can_substitute_null_values()
        {
            MapperConfiguration Configuration = new MapperConfiguration(cfg =>
          {
              cfg.CreateMap<Source, Dest>().ForMember(m => m.ValuePropertyNotMatching, opt =>
              {
                  opt.MapFrom(src => src.Value);
                  opt.NullSubstitute("5");
              });

          });

            var source = new[] { new Source { Value = null } }.ToList();
            _dests = Configuration.CreateMapper().Map<List<Dest>>(source);
            Assert.Equal("5", _dests[0].ValuePropertyNotMatching);


        }
    }
}

It looks a bit like what was described in https://github.com/AutoMapper/AutoMapper/issues/642