Seeing a black screen on an ofxCvGrayscaleImage

Hi all,
I have this bit of code

update(){
    ....
    ....
    if ( bUndistort && bIsUndistortionAvailable )
    {
        calibrateCamera.undistort( toCv( grayCameraFrame ) , toCv( undistortedGrayFrame ) );
    }
    else
    {
        undistortedGrayFrame = grayCameraFrame;
    }
    undistortedGrayFrame.flagImageChanged();
}

I also have the following class methods to expose “grayCameraFrame” and “undistortedGrayFrame”

namely:

    ofxCvGrayscaleImage& getGSFrame() { return grayCameraFrame ;}
    ofxCvGrayscaleImage& getUGSFrame() { return undistortedGrayFrame ;}

So from another class, I can ask for either of these 2 images.

I can get the grayscale image fine, but the undistortedGrayScaleFrame is just black. Am I forgeting something? Why is the undistorted image only black?

thank you in advance for your help!