Xamarin Binding Objective-C - Unrecognized selector sent

I am having a problem binding this protocol method.

- (void)onInviteTrying:(long)sessionId;

      

        
        
        
      

    

Seems pretty simple, but it looks like my C # method was not found. Here is what I have in the ApiDefinition.cs file

[BaseType (typeof (NSObject))]
[Protocol]
[Model]
interface PortSIPEventDelegate {

    [Export ("onInviteTrying:")]
    void onInviteTrying (int sessionId);

    //
    // more stuff here
    //

}

interface IPortSIPEventDelegate {}

//
// more stuff here
//

      

        
        
        
      

    

and here is my C # file ...

public class IOSSIPManager : NSObject, IPortSIPEventDelegate, ISIPClient {
    //
    // more stuff here
    //
    public void onInviteTrying(int sessionId)
        {
            // An invite is first trying.
            this.OnCallStatusChanged(sessionId, CallStatusUpdate.InviteTrying);

            ConnectionStatusManager.Instance.Model.SIPDetailedStatus = "SIP OnInviteTrying";
        }
    }

      

        
        
        
      

    

I am getting the following error

Objective-C exception. Name: NSInvalidArgumentException Cause: - [StreetSupervisorIOS_IOSSIPManager onInviteTrying:]: unrecognized selector posted to instance 0x17011e50

Any idea what I did wrong? My other methods are working fine. Maybe the problem is with the long ones? I tried with longs and IntPtrs instead of ints, but it didn't work. Thanks in advance.