I trying to update nested collection (Cities) of Country entity.

Just simple enitities and dto's:

// EF Models
public class Country
{
    public int Id { get; set; }
    public string Name { get; set; }
    public virtual ICollection Cities { get; set; }
}

public class City
{
    public int Id { get; set; }
    public string Name { get; set; }
    public int CountryId { get; set; }
    public int? Population { get; set; }

    public virtual Country Country { get; set; }
}

// DTo's
public class CountryData : IDTO
{
    public int Id { get; set; }
    public string Name { get; set; }
    public virtual ICollection Cities { get; set; }
}

public class CityData : IDTO
{
    public int Id { get; set; }
    public string Name { get; set; }
    public int CountryId { get; set; }
    public int? Population { get; set; }
}

And code itself (tested in console app for the sake of simplicity):

        using (var context = new Context())
        {
            // getting entity from db, reflect it to dto
            var countryDTO = context.Countries.FirstOrDefault(x => x.Id == 1).ToDTO();

            // add new city to dto 
            countryDTO.Cities.Add(new CityData 
                                      { 
                                          CountryId = countryDTO.Id, 
                                          Name = "new city", 
                                          Population = 100000 
                                      });

            // change existing city name
            countryDTO.Cities.FirstOrDefault(x => x.Id == 4).Name = "another name";

            // retrieving original entity from db
            var country = context.Countries.FirstOrDefault(x => x.Id == 1);

            // mapping 
            AutoMapper.Mapper.Map(countryDTO, country);

            // save and expecting ef to recognize changes
            context.SaveChanges();
        }

This code throws exception:

The operation failed: The relationship could not be changed because one or more of the foreign-key properties is non-nullable. When a change is made to a relationship, the related foreign-key property is set to a null value. If the foreign-key does not support null values, a new relationship must be defined, the foreign-key property must be assigned another non-null value, or the unrelated object must be deleted.

even though entity after last mapping seems just fine and reflects all changes properly.

I've spent much time finding solution but got no result. Please help.