Cannot convert lambda expression to type 'string' because it is not a delegate type

In my controller i am trying to use include with EF4 to select related entities, but the lambda expression is throwing the following error,

i have the related entity defined in the Entity class like

public class CustomerSite
{
    public int CustomerSiteId { get; set; }
    public int CustomerId { get; set; }
    public virtual Customer Customer { get; set; }
}

Then in my controller i have

 var sites = context.CustomerSites.Include(c => c.Customer);

 public ViewResult List()
 {
    var sites = context.CustomerSites.Include(c => c.Customer);
    return View(sites.ToList());
 }

Can anyone kindly point me in the right direction on what i'm doing wrong here?