Themidibus - can't receive input

Hello everyone,
I’m trying to connect my digital piano to processing, with the themidibus library. I have no problem for sending messages to my piano but I can’t receive any information from it. I used the example code from the Github page (themidibus/Basic.pde at master · sparks/themidibus · GitHub) and didn’t change anything except one thing : I replaced the line 24
myBus = new MidiBus(this, -1, “Java Sound Synthesizer”);
with
myBus = new MidiBus(myBus, 1, 4);
where 1 and 4 correspond respectively to my piano as input device and output device. I had to replace “this” by “myBus” otherwise I had a null pointer exception.
How is it possible that I can send outputs but not receive any input ?
As you can see from my message, I’m really new to this field so please take it easy ^^
Thank you very much for reading and hope you can help me !