System.ArgumentException: Value does not fall within the expected range C# when subscribing to event in a different class

Question

I'm developing an Android C# Application using Xamarin. It has a very simple structure: the MainActivity launches and creates instances of classes GUIHandler and Connection. Each class raises different events:

In Connection.cs:

    public EventHandler<ConnectionEventArgs> ConnectionMade;
    protected virtual void OnConnectionMade()
    {
        if (ConnectionMade != null) {
            ConnectionMade (this, new ConnectionEventArgs ());
        }
    }
    public EventHandler<ConnectionEventArgs> ConnectionFailed;
    protected virtual void OnConnectionFailed()
    {
        if (ConnectionFailed != null) {
            ConnectionFailed (this, new ConnectionEventArgs ());
        }
    }
    public EventHandler<ConnectionEventArgs> LostConnection;
    protected virtual void OnLostConnection()
    {
        if (LostConnection != null) {
            LostConnection (this, new ConnectionEventArgs ());
        }
    }
    public EventHandler<ConnectionEventArgs> ReceivedData;
    protected virtual void OnReceivedData(byte[] _byteDataReceived, int _byteDataReceivedLength)
    {
        if (ReceivedData != null) {
            ReceivedData (this, new ConnectionEventArgs {byteDataReceived = _byteDataReceived, byteDataReceivedLength = _byteDataReceivedLength});
        }
    }
    public EventHandler<ConnectionEventArgs> SentData;
    protected virtual void OnSentData()
    {
        if (SentData != null) {
            SentData (this, new ConnectionEventArgs ());
        }
    }

In GUIHandler.cs:

    public EventHandler<GUIHandlerEventArgs> SendButtonClicked;
    protected virtual void OnSendButtonClicked()
    {
        if (SendButtonClicked != null) {
            SendButtonClicked (this, new GUIHandlerEventArgs { sendTextEditData = sendEditText.Text });
        }
    }

The subscriptions are in the MainActivity.cs file, which gets run when the application is executed:

    ...
    protected override void OnCreate (Bundle bundle)
    {
        base.OnCreate (bundle);
        SetContentView (Resource.Layout.Main);

        var guiHandler = new GUIHandler();
        var connection = new Connection();

        connection.ConnectionMade += guiHandler.OnConnectionMade;
        connection.ConnectionFailed += guiHandler.OnConnectionFailed;
        connection.LostConnection += guiHandler.OnLostConnection;
        connection.ReceivedData += guiHandler.OnReceivedData;
        connection.SentData += guiHandler.OnSentData;

        // THE CODE BELOW RAISES System.ArgumentException: Value does not fall within expected range DURING RUNTIME.
        guiHandler.SendButtonClicked += connection.OnSendButtonClicked;
        // However, this line makes the job.
        guiHandler.SendButtonClicked += (sender, args) => connection.Send (args.sendTextEditData);
    }

I can't find the reason why it raises that Exception there and does not anywhere else. However, doing some debugging I noticed I can't subscribe a public method from connection, but have no idea why. Anyone has any idea?

Edit: this is the definition of OnSendButtonClicked in Connection.cs:

    public void OnSendButtonClicked (object sender, GUIHandlerEventArgs args)
    {
        //Send data
        Send(args.sendTextEditData);
    }