I have tried everything I could think of with no luck to speak of. What I am attempting to do is very simple. I would like to be able to put up a message box with an error and allow the user to enter data ( in the event that the user clicks on the calculate button without any data in the form. I am halfway there, but run into a strong wall in getting the code right. I welcome any and all helpful comments and or suggestions.

Code:
Public Class LongDistanceCall

    Dim notANumber As Boolean
    Dim DAYRATE As Double = 0.07
    Dim EVENRATE As Double = 0.12
    Dim OFFPEAK As Double = 0.05
    Dim MinutesUsed As Double

    Private Sub btnCalculate_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnCalculate.Click

 ' ********************* start of problem area   **********************************
        MinutesUsed = CDbl(txtMinutesUsed.Text)

        If (MinutesUsed) <= 0 Then
            MessageBox.Show("Can not less then 1 Minute", "ERROR DETECTED", MessageBoxButtons.OK, MessageBoxIcon.Error)
            txtMinutesUsed.Text = String.Empty
        End If

 ' ********************* end of problem area   **********************************

        If RadDayTime.Checked = True Then
            lblCharges.Text = FormatCurrency(MinutesUsed * DAYRATE)
        ElseIf RadEvening.Checked = True Then
            lblCharges.Text = FormatCurrency(MinutesUsed * EVENRATE)
        ElseIf RadOffPeak.Checked = True Then
            lblCharges.Text = FormatCurrency(MinutesUsed * OFFPEAK)
        Else
            MessageBox.Show("Pleae Select one of the offered plans", "Missing information", MessageBoxButtons.OK, MessageBoxIcon.Information)

        End If


    End Sub

    Private Sub btnClear_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnClear.Click

        lblCharges.Text = String.Empty
        RadDayTime.Checked = False
        RadEvening.Checked = False
        RadOffPeak.Checked = False
        txtMinutesUsed.Clear()

    End Sub

    Private Sub btnExit_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnExit.Click
        Me.Close()
        End

    End Sub