สอบถามเรื่องการกรอกค่าแล้ว Error ครับ ควรจะแก้ไขอย่างไรดีครับ

Public Class Form1
    Dim a, b, c As Double

    Private Sub TextBox1_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles TextBox1.TextChanged
        a = TextBox1.Text
    End Sub

    Private Sub TextBox2_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles TextBox2.TextChanged
        b = TextBox2.Text
        b = CDbl(b)
    End Sub

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        c = a + b
        TextBox3.Text = c
    End Sub
End Class