Unable to invoke PeopleSoft Web Service utilizing C#.NET

I'm trying to connect a C# app to a PeopleSoft WSDL web service. I've been through the standard processes of adding the service reference, and fought some battles to get the Username/Password included in the request. That's all working, but when the response comes back, I end up with a exception trying to parse the response:

System.ServiceModel.Security.MessageSecurityException: Security processor was unable to find a security header in the message. This might be because the message is an unsecured fault or because there is a binding mismatch between the communicating parties. This can occur if the service is configured for security and the client is not using security.