RPi - error using ofPixels

hello,

I am having an error using a raspberry pi 2 under Raspbian PIXEL with oF 0.9.8
I am using a piCamera module v2 with

Everything is compiling ok. App runs but when trying to save a part of a grabbed image i get an error :

[ error ] ofPixels: image type not supported

Error is thrown in the Composer::add() function as follow :

//ofApp
 ofPixels img;
 img = preview.grabCroppedFrame();
 compose.add(img, word.pickedText);


//composer class
vector<ofPixels> images;    

void Composer::add(ofPixels image) {
 // ERROR SITS HERE !!
    images.push_back(image);
}

void Composer::create() {
    ofImage img;
    img.setFromPixels(images.at(0));
}

Everything works fine using a webcam instead of using the piCamera