I am trying to draw a line on a form with the following code, but I get a strange error....

Code:
private void button2_Click(object sender, System.EventArgs e)
		{
			Graphics g;
			Pen aPen = new Pen(Brushes.Black, 2);
			g.DrawLine(aPen, 100,100,300,300);
			
			
		}
Error:
C:\Documents and Settings\carsten\My Documents\Visual Studio Projects\sharp_test_obel\Form1.cs(228): Use of unassigned local variable 'g'

What am I doing wrong?