Automapper 5.2.0 and projections with EF6 throws Exception - bug?

Question

When using Automapper 3.3.1 or 4.2.1 my code is running fine.

But after upgrade to 5.2.0 I get ArgumentException: "Type 'System.String' does not have a default constructor" in AutoMapper.QueryableExtensions.ExpressionBuilder.DestinationConstructorExpression.

We have a legacy database that supports several languages and in our EF6 model we us a LocalizedString complex type to hold all the translated values for each string property. Based on this article.

Sample code:

    static void Main(string[] args)
    {
        Mapper.Initialize(p =>
            {
                CreateCustomMappingForLocalizedString(p);
                p.CreateMap<Country, CountryViewModel>();
            }
        );

        var db = new DemoContext();
        var countries = db.Country.ProjectTo<CountryViewModel>(new {lang = "nb"}).ToList();

        foreach (var c in countries)
            Console.WriteLine($"{c.CountryId} : {c.CountryCode} : {c.CountryName}");

        Console.WriteLine("Press <ENTER> to continue");
        Console.ReadLine();
    }

And the custom mapping of LocalizedString is

    public static void CreateCustomMappingForLocalizedString(IMapperConfigurationExpression cfg)
    {
        string lang = null;
        // Create MAP for LINQ Projections
        cfg.CreateMap<LocalizedString, string>()
            .ProjectUsing(src =>
            lang == "en"? src.ValueEn :
            lang == "da"? src.ValueDa :
            lang == "nn"? src.ValueNnNo :
            lang == "sv"? src.ValueSv :
            lang == "de" ? src.ValueDe :
            lang == "fi"? src.ValueFi :
            lang == "se"? src.ValueSeNo :
            src.ValueNo);
        cfg.CreateMap<LocalizedString, string>()
          .ConvertUsing(src =>
            Thread.CurrentThread.CurrentCulture.TwoLetterISOLanguageName == "en" ? src.ValueEn :
            Thread.CurrentThread.CurrentCulture.TwoLetterISOLanguageName == "da" ? src.ValueDa :
            Thread.CurrentThread.CurrentCulture.TwoLetterISOLanguageName == "nn"? src.ValueNnNo :
            Thread.CurrentThread.CurrentCulture.TwoLetterISOLanguageName == "sv"? src.ValueSv :
            Thread.CurrentThread.CurrentCulture.TwoLetterISOLanguageName == "de"? src.ValueDe :
            Thread.CurrentThread.CurrentCulture.TwoLetterISOLanguageName == "fi"? src.ValueFi :
            Thread.CurrentThread.CurrentCulture.TwoLetterISOLanguageName == "se"? src.ValueSeNo :
            src.ValueNo);
    }

The only change in moving to 5.2.0 from 4.2.0 is to change parameter type in CreateCustomMappingForLocalizedString from IMapperConfiguration to IMapperConfigurationExpression.

Am I doing something wrong here with Automapper 5.2.0 or is this a bug in Automapper?

I have put a small sample solution with separate projects for versions 3.3.1, 4.2.1 and 5.2.0 on my GitHub repo.