Azure Mobile Service Query Help

What is wrong wit the following query using Mobile Service. It just throws an exception.

public async Task GetContactAsync(string identifiername)
{

        List<Contact> items = await ContactTable
              .Where(o => o.IdentityProviderName == identifiername)
              .ToListAsync();

        return items.Count == 0 ? null : items[0];
    }

I am getting the following error.

Unhandled Exception:

System.ArgumentNullException: Value cannot be null.
Parameter name: cancel