The given key was not present in the dictionary.

When i am using WCF with Silverlight then this exception "

 The given key was not present in the dictionary.

"  comes why?

Here code of user contro load where exception comes:

 private void UserControl_Loaded(object sender, RoutedEventArgs e)
        {
            ServiceReference1.Service1Client client = new MySilverlightApp.ServiceReference1.Service1Client();

i am using Silverlight 3 SDK and Visual Studio 2008