Unknown Exeption When I Draw Line Using System.draw

what this exeption for (Parameter is not valid)

my code to draw line is 

Imports System.Drawing

Imports System.Drawing.Drawing2D

Imports System.Drawing.Imaging

Imports System.Drawing.Text



Public Class Form1

    Public myPen As New Pen(Color.Azure, 15)

    

    Public x1, y1, x2, y2 As Integer

    Public gr As Graphics

    Public Sub Form1_Paint(ByVal sender As Object, ByVal e As PaintEventArgs) Handles MyBase.Paint



        gr = e.Graphics



        gr.DrawLine(myPen, x1, y1, x2, y2)



    End Sub



    Private Sub Form1_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load

        myPen.DashStyle = DashStyle.DashDot

    End Sub



    Private Sub Form1_MouseDown(ByVal sender As Object, ByVal e As _ System.Windows.Forms.MouseEventArgs) Handles Me.MouseDown

        x1 = e.X : y1 = e.Y



    End Sub



    Private Sub Form1_MouseUp(ByVal sender As Object, ByVal e As _ System.Windows.Forms.MouseEventArgs) Handles Me.MouseUp

        x2 = e.X : y2 = e.Y





        gr.DrawLine(myPen, x1, y1, x2, y2)







    End Sub



    





End Class