Tileing image on JPanel, java

I have a method to set the 'texture' of a JPanel, however it is throwing a NullPointerException, and i cannot figure out why.

Method:

void setTexutre(Image tileImage) {
    Graphics g = panel.getGraphics();
    int width = (int) getBounds().getWidth();
    int height = (int) getBounds().getHeight();
    int imageW = tileImage.getWidth(panel);
    int imageH = tileImage.getHeight(panel);
    for (int x5 = 0; x5 < width; x5 += imageW) {
        for (int y5 = 0; y5 < height; y5 += imageH) {
            g.drawImage(tileImage, x5, y5, panel);
        }
    }
    panel.paint(g);
}

The NullPointerException is thrown when i call "g.drawImage(tileImage, x5, y5, panel);"

and yes, the image is a real image, i have checked. In the wethod above "panel" is defined as a new JPanel, and intializes normally when i do not call the method.

Thanks for any help!