DbContext Disposed after the first request when using Ninject InRequestScope ()

I'm new to EF and Ninject, so forgive me if that doesn't make sense :)

I have an MVC3 application with Ninject and Ninject.Web.Common links. I am trying to inject DbContext into my repositories. I see that on the first request everything works wonderfully, but subsequent requests are returned:

System.InvalidOperationException: The operation cannot be completed because the DbContext has been disposed.
   at System.Data.Entity.Internal.LazyInternalContext.InitializeContext()
   at System.Data.Entity.Internal.Linq.DbQueryProvider.Execute[TResult](Expression expression)
   at System.Linq.Queryable.SingleOrDefault[TSource](IQueryable`1 source, Expression`1 predicate)

My bindings:

kernel.Bind<ISiteDataContext>().To<SiteDataContext>().InRequestScope();
kernel.Bind<IProductRepository>().To<ProductRepository>();
kernel.Bind<IProductService>().To<ProductService>();

My class of service:

public class ProductService : IProductService {
    [Inject]
    public IProductRepository repository {get; set;}

    ...
}

My repository class:

public class ProductRepository : IProductRepository {
    [Inject]
    public ISiteDataContext context {get; set;}

    ...
}

My SiteDataContext class:

public class SiteDataContext  : DbContext, ISiteDataContext 
{
    static SiteDataContext()
    {
        Database.SetInitializer<SiteDataContext >(null);
    }

    public DbSet<Product> Products{ get; set; }


    protected override void Dispose(bool disposing)
    {
        base.Dispose(disposing);
    }
}

My controller:

public class ProductController {
    [Inject]
    public IProductService productService {get; set;}

    ...
}

If I delete .InRequestScope (), then it works fine, but then it causes problems with the Entity Framework, because the objects change in several separate instances of the data context.