title: Missing compiler required member 'System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1.AwaitOnCompleted'
body: <p>After adding this method (first test on C# async keyword in my life) to my project I get the error below, why?</p>
  
  <pre><code>public async Task&lt;Member&gt; 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);
  
     return (await mailChimpManager.Members.AddOrUpdateAsync(ConfigurationManager.AppSettings["mailChimpDownloadListID"], member));
  }
  </code></pre>
  
  <p>Error inside VStudio 2015 after compilation:</p>
  
  <blockquote>
    <p>Missing compiler required member 'System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1.AwaitOnCompleted'</p>
  </blockquote>
  
  <p>My project is a ASP.NET MVC 4 targeting .NET Framework 4.5 (just upgraded from 4.0) and I'm trying to integrate <a href="https://github.com/brandonseydel/MailChimp.Net" rel="nofollow">MailChimp.NET v3</a> inside it.</p>
  
  <p>Thanks.</p>
  
  <p><strong>EDIT:</strong> Created a brand new MVC WebApplication .NET 4.5 project in VStudio 2015 and the same method compiles without errors. What should I check?</p>