Sharpen Image with Java

I use this Method to sharpen an Image. But i m gotting error. ("unable to convolve src Image")
public BufferedImage changeSharpness(BufferedImage img, int sharpen)
    {
        try{
        for(int i=0;i<sharpen;i++){
       
        Kernel kernel = new Kernel(3, 3,
        new float[] {
            -1, -1, -1,
            -1, 9, -1,
            -1, -1, -1});

        BufferedImageOp op= new ConvolveOp(kernel);
        buff = op.filter(img,null);
        }
        }
        catch(Exception e)
        {
            JOptionPane.showMessageDialog(new JFrame(),e.getMessage(),"Test",0);
        }
        return buff;
    }