Error in EslClient.EventNotificationService.Register

When calling the following register function :

EslClient eslClient = new EslClient(apikey, apiUrl);
eslClient.EventNotificationService.Register(
                EventNotificationConfigBuilder.NewEventNotificationConfig(callBackUrl)
                .ForEvent(NotificationEvent.SIGNER_COMPLETE).ForEvent(NotificationEvent.PACKAGE_DELETE));

I am getting: System.NullReferenceException: Object reference not set to an instance of an object. This is happening on our development machines but our actual live application is running just fine, this error just propped up today. Everything has been working fine for months so we are kind of stuck at the moment on what the issue could be. Let me know if there's other info I can post that would help, thanks.