Checkbox + Textbox run แล้วติด โปรแกรมแจ้ง Conversion from string "" to type 'Integer' is not valid.

        Dim tea, greentea, coco, coffee As String
       
        Dim pay, total, change As Integer
        
        Dim result As String

        tea = CInt(teabox.Text)
        greentea = CInt(greenteabox.Text)
        coco = CInt(cocobox.Text)
        coffee = CInt(coffeebox.Text)
        pay = CInt(paybox.Text)
        change = CInt(changebox.Text)
        total = CInt(totalbox.Text)

        totalbox.Text = CInt(teabox.Text) + CInt(greenteabox.Text) + CInt(cocobox.Text) + CInt(coffeebox.Text)
        changebox.Text = CInt(paybox.Text) - CInt(totalbox.Text)

        Int(total = 0)
        result = ""
        If (teachackbox.Checked = True) Then
            total += 30 + tea
            result += teachackbox.Text + "\n"
        End If

        If (greenteacheckbox.Checked = True) Then
            total += 30 + greentea
            result += greenteacheckbox.Text + "\n"
        End If

        If (cococheckbox.Checked = True) Then
            total += 35 + coco
            result += cococheckbox.Text + "\n"
        End If

        If (coffeecheckbox.Checked = True) Then
            total += 35 + coffee
            result += coffeecheckbox.Text + "\n"

        End If