VB.Net - Conversion from string "" to type 'Single' is not valid. แก้ยังไงครับ

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click

        If TxtAmount.Text.Trim = "" Then
            MessageBox.Show("กรุณาใส่จำนวน")
            TxtAmount.Focus()
            Exit Sub
        End If

        If TxtAmount.Text = 0 Then
            MessageBox.Show("กรุณาใส่จำนวนที่ไม่ใช่ 0")
            TxtAmount.Focus()
            Exit Sub
        End If

        If CInt(TxtAmount.Text) > CSng(unit) Then
            MessageBox.Show("สินค้าไม่พอ เหลือ" & unit & "เท่านั้น")
            TxtAmount.Focus()
            TxtAmount.SelectAll()
            Exit Sub
        End If

        For i As Integer = 0 To ListView1.Items.Count - 1
            If TxtProid.Text = ListView1.Items(i).SubItems(1).Text Then
                MessageBox.Show("รหัสซ้ำ")
                Exit Sub
            End If
        Next

                Dim un As Integer = 0 'แสดงจำนวนคงเหลือ
                un = unit - CInt(TxtAmount.Text)

                Dim sum As Single = 0.0
                sum = CSng(TxtSalePrice.Text) * CInt(TxtAmount.Text) * 7 / 100 + CSng(TxtSalePrice.Text) 'CIntข้อความเป็นจำนวนเต็ม  CSng= แปลงข้อความเป็นตัวเลข

                num += 1
                Dim anydata() As String
                anydata = New String() {num, TxtProid.Text, TxtProName.Text, TxtSalePrice.Text, TxtAmount.Text, sum, un}
                Dim lv As New ListViewItem(anydata)
                ListView1.Items.Add(lv)
                lv = Nothing 'โค้ตดักค่าว่าง'

        Dim Total As Integer
        Total = TxtTotal.Text
        TxtTotal.Text = (CSng(TxtTotal.Text) + sum).ToString("#,##")


    End Sub