C# ASPX - cannot be modified while it is in the Opened state.

I have a C# ,net ASPX program that produces this error sometimes when trying to connect to a web service.  What does this mean and how can I prevent?  Does this occur because the prevent execution did not close this state?

The communication object, System.ServiceModel.ChannelFactory`1[TestService.ServiceReference1.stepPortType], cannot be modified while it is in the Opened state.

Client.Endpoint.Address = new System.ServiceModel.EndpointAddress(ServerName);
                Client.Open();
                return Client;