Missing compiler required member 'System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1.AwaitOnCompleted'

After adding this method (first test on C# async keyword in my life) to my project I get the error below, why?

public async Task<Member> AddOrUpdate(DownloadFormModel model)
{
   IMailChimpManager mailChimpManager = new MailChimpManager(ConfigurationManager.AppSettings["mailChimpKey"]);

   Member member = new Member {EmailAddress = model.Email, Status = Status.Subscribed};
   member.MergeFields.Add("NAME", model.Name);

   retu (await mailChimpManager.Members.AddOrUpdateAsync(ConfigurationManager.AppSettings["mailChimpDownloadListID"], member));
}

Error inside VStudio 2015 after compilation:

Missing compiler required member 'System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1.AwaitOnCompleted'

My project is a ASP.NET MVC 4 targeting .NET Framework 4.5 (just upgraded from 4.0) and I'm trying to integrate MailChimp.NET v3 inside it.

Thanks.