Problem with Do Loop

[h0neydip]

This code suppose to take the user's input of current balnace, the annual interest rate, the current month, and amount he/she will pay per month. When the CALCULATE button is clicked the program starting with the current month, should compute and display for each month until the balance is reduced to zero, the month, the interest charge, payment, and new balance. But somehow im stuck in an infinite loop and its not displaying anything. please help me

Public Class frmPaySchedule

    Private Sub btncalculate_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btncalculate.Click

        
        Dim month As Integer = CInt(txtmonth.Text)
        Dim balance As Decimal = CDec(txtbalance.Text)
        Dim interest As Decimal = CDec(((txtAPR.Text / 100) / 12) * balance)
        Dim payment As Decimal = CDec(txtpayment.Text)

        Do Until balance = 0
            If month = 5 Or month = 6 Or month = 7 Or month = 8 Then
                payment = payment + 100
            End If
            If month = 11 Then
                payment = payment + 50
            End If
            balance = balance - (payment - interest)
            txtResult.Text = month.ToString & " " & FormatCurrency(interest) & " " & _
                             FormatCurrency(payment) & " " & FormatCurrency(balance)

            month += 1

        Loop



    End Sub
End Class