ColourClustering

getting error for getRGB() and i dont know why?

package colourclustering;

import java.awt.Image;
import java.awt.Toolkit;
import java.util.HashMap;

/**
 *
 * @author u0207372
 */

public class ColourClusterTester {
    private HashMap hashMap;
    private Image img;
   
    /**
     * Creates a new instance of ColourClusterTester
     */
    public ColourClusterTester( String filename ) {
       
        hashMap = new HashMap();
        img = Toolkit.getDefaultToolkit().getImage( filename );
       
        for ( int i = 0; i < img.getWidth(null); i++ )
        {
            for ( int j = 0; j < img.getHeight(null); j++ )
            {
                int pixelFreq;
                int pixelValue = img.getRGB();
               
                hashMap.put( pixelValue, 1 );
            }
        }
    }
   
    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        new ColourClusterTester( "H:\\MIOCReader\\ColourClustering\\ColourClustering\\src\\colourclustering\\test.gif" ) ;
    }
   
}

bascially what im trying to do is read image, get pixel value of each pixel and store value in hash map. along with the number of time that value appears but having some problems actually getting the RGB() for each pixel?