Unable to return custom class from WCF Data Service

Question

I am trying to return a custom class from my WCF data service. My custom class is:

[DataServiceKey("ID")]
public class Applist {
    public int ID { get; set; }
    public string Name { get; set; }
}

My data service looks like:

public static void InitializeService(IDataServiceConfiguration config)
{
    config.RegisterKnownType(typeof(Applist));
    config.SetEntitySetAccessRule("*", EntitySetRights.All);
    config.SetServiceOperationAccessRule("GetApplications", ServiceOperationRights.AllRead);
}

[WebGet]
public IQueryable<Applist> GetApplications() {
    var result = (from p in this.CurrentDataSource.Applications
          orderby p.ApplicationName
          group p by p.ApplicationName into g
          select new Applist { ID = g.Min(p => p.id), Name = g.Key });

    return result.AsQueryable();
}

However when I run the service, it gives me an error:

Request Error Request Error The server encountered an error processing the request. 
The exception message is 'Unable to load metadata for return type
'System.Linq.IQueryable`1[ApplicationService.Applist]' of method
'System.Linq.IQueryable`1[ApplicationService.Applist] GetApplications()'

The same query runs perfectly fine in LINQPad.