GUI why not working?

Why is this not working? It's two classes, one is the main program and one is the GUI. When I compile it, it say's that it can't find "class GUI"...

CODE:
package progprojekt;

import javax.swing.*;
import java.awt.*;
import java.util.*;

public class Waldo {


    public Waldo () {

        //GUI gui = new GUI (this);

        JFrame waldo = new JFrame ("Title");
        waldo.getContentPane().add (new GUI());
        waldo.setSize (800, 600);
        waldo.show();
    }

    public static void main (String[] args) {

        Waldo waldo = new Waldo();



    }
}

-----------------------------------------------------------
package progprojekt;

import javax.swing.*;
import java.awt.*;


public class GUI extends JPanel {


    public GUI() {

        setBackground (Color.orange);
        setLayout (null);

    }
}