How to check customer record already exist in NAV ?

Hi expert ,

   I have been successfully added customer using WebService.Now i want to check  customer  in NAV after that  update these customer . I have used  following code

         For added :

                   Customer oNavServiceCustomer = new Customer();

                   Customer_Binding oNavServiceCustomerBinding == new Customer_Binding();

                   oNavServiceCustomerBinding.UseDefaultCredentials = true;

                    oNavServiceCustomer.No = "C1" ;

                    oNavServiceCustomer.Name = "Surajit Kundu";

                    oNavServiceCustomerBinding.Create(ref oNavServiceCustomer);

          For updated :  before update i want to check it  in NAV  for that i used below code

                     if (!string.IsNullOrEmpty( oNavServiceCustomerBinding.GetRecIdFromKey(oNavServiceCustomer.No)  ) )

                             its return a error message 'Key "C1" is not well formed!'

                     why this error  is reurn ? 

                   Please give me any suggestion how to check customer  in NAV?

Thanks 

Surajit