C# - How To Make The Clause Different

I have the following code:

CustomerService service;
public CustomerService Service
{
get
{
if (this.service == null)
{
this.service = new CustomerService();
}
return this.service;
}
}
public DataTable GetCustomers()
{
return this.Service.GetCustomers();
}

Now the question is: if I wrote the above method as follow (without "this"), it's giving me an error : instance is not reference to an object.

public DataTable GetCustomers()
{
return Service.GetCustomers(); // this will spell the error "instance is not reference to an object"
}

Does anyone know? also it only happens while running via IIS and not from casini web server (VS 2010).