Using AutoMapper to Map IList<TSource> to (Iesi.Collections.Generic) ISet<TDestination>

I have been trying to solve this issue for a day now and have got no where so am hoping someone might have solved this before. The closest I found to a solution was How to simply map an NHibernate ISet to IList using AutoMapper and Map IList to ICollection through AutoMapper but still no joy.

I have a data object that looks like:

public class Parent
{
   public virtual ISet<Child> Children  {get; set; }
}

And a business object that looks like:

public class ParentDto
{
   public IList<ChildDto> Children  {get; set; }
}

Using AutoMapper to map from Data to Business works fine:

...
Mapper.CreateMap<Parent, ParentDto>();
Mapper.CreateMap<Child, ChildDto>();
...

ParentDto destination = CWMapper.Map<Parent, ParentDto>(source);

But when I come to Mapping from Business to Data I get the error:

...
Mapper.CreateMap<ParentDto, Parent>();
Mapper.CreateMap<ChildDto, Child>();
...

Parent destination = CWMapper.Map<ParentDto, Parent>(source);

Unable to cast object of type 'System.Collections.Generic.List' to ''Iesi.Collections.Generic.ISet'

I added a custom mapping:

Mapper.CreateMap<ParentDto, Parent>()
      .ForMember(m => m.Children, o => o.MapFrom(s => ToISet<ChildDto>(s.Children)));

private static ISet<T> ToISet<T>(IEnumerable<T> list)
    {
        Iesi.Collections.Generic.ISet<T> set = null;

        if (list != null)
        {
            set = new Iesi.Collections.Generic.HashedSet<T>();

            foreach (T item in list)
            {
                set.Add(item);
            }
        }

        return set;
    }

But I still get the same error. Any help would be greatly apriciated!