Java Swing White Gap Under Title Bar

I am creating a simple Java GUI with Swing and I have a white bar that appears under the title bar that I can't seem to figure out how to get rid of. I appreciate any help and tips as I have searched all over for anyone with a similar problem and have not been able to find anything. Thanks

Image of Java GUI:

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

public class ja {

    public static void main(String[] args) {
            JFrame f = new JFrame("jA");
            JPanel p = new JPanel();
            p.setLayout(new FlowLayout());
            f.setSize(400, 600);
            f.setLocationRelativeTo(null);
            f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

            JLabel bg = new JLabel(new ImageIcon("brown.jpg"));
            bg.setLayout(new FlowLayout());
            p.add(bg);

            JButton b1 = new JButton("b1");
            JButton b2 = new JButton("b2");
            JButton b3 = new JButton("b3");

            bg.add(b1);
            bg.add(b2);
            bg.add(b3);

            f.add(p);

            f.setVisible(true);
    }
}