Mapping GroupBy.SelectMany Expression throws a System.InvalidOperationException.

[BlaiseD]

Steps to reproduce:

    class Program
    {
        static void Main(string[] args)
        {
            var config = new MapperConfiguration
           (
                cfg =>
                {
                    cfg.AddExpressionMapping();

                    cfg.CreateMap<User, UserModel>()
                     .ForMember(d => d.Id, opt => opt.MapFrom(s => s.UserId))
                     .ForMember(d => d.FullName, opt => opt.MapFrom(s => string.Concat(s.FirstName, " ", s.LastName)))
                     .ForMember(d => d.AgeInYears, opt => opt.MapFrom(s => s.Age));
                }
           );
           var mapper = new Mapper(config);

            Expression<Func<IQueryable<UserModel>, IQueryable<UserModel>>> exp = q => q.OrderBy(s => s.Id).ThenBy(s => s.FullName).GroupBy(s => s.AgeInYears).SelectMany(grp => grp);

            Expression<Func<IQueryable<User>, IQueryable<User>>> expMapped = mapper.Map<Expression<Func<IQueryable<User>, IQueryable<User>>>>(exp);
        }
    }

    public class User
    {
        public int UserId { get; set; }
        public string FirstName { get; set; }
        public string LastName { get; set; }
        public int Age { get; set; }
        public bool Active { get; set; }
    }

    public class UserModel
    {
        public int Id { get; set; }
        public string FullName { get; set; }
        public string AccountName { get; set; }
        public int AgeInYears { get; set; }
        public bool IsActive { get; set; }
    }

Error:

AutoMapper.AutoMapperMappingException
  HResult=0x80131500
  Message=Error mapping types.
  Source=<Cannot evaluate the exception source>
  StackTrace:
<Cannot evaluate the exception stack trace>

Inner Exception 1:
InvalidOperationException: No generic method 'SelectMany' on type 'System.Linq.Queryable' is compatible with the supplied type arguments and arguments. No type arguments should be provided if the method is non-generic.