"Index and length must refer to a location within the string. Parameter name: length"

[stuart1512]

i am creatign a program to get a duration from the user and the amound per second. it calculates the price per second for a phone call but i have an issue with 2 lines

varSeconds = CType(tbCallLength.Text.Substring(Sep2, 2), Double)
and
varPence = CType(tbCallPrice.Text.Substring(Sep4, 2), Double)

with this error message "Index and length must refer to a location within the string. Parameter name: length"

can u help please, here is the whole of my code

Option Strict On
Public Class frmCallCalculator

    Dim Sep1, Sep2, Sep3, Sep4 As Integer
    Dim varMinutes, varSeconds As Double
    Dim varCal1, varCal2 As Double
    Dim varPounds, varPence As Double

    Private Sub btnCalculatePrice_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnCalculatePrice.Click

        Call Minutes()
        Call Seconds()
        Call Pounds()
        Call Pence()

        varCal1 = CType(varSeconds + (varMinutes * 60), Double)
        varCal2 = varPence + (varPounds * 100)

        tbAnswer.Text = CType(varCal1 * varCal2, String)
    End Sub

    Private Sub Minutes()
        Sep1 = InStr(1, tbCallLength.Text, ":")
        Sep1 = Sep1 - 1
        varMinutes = CType(tbCallLength.Text.Substring(0, Sep1), Double)
    End Sub

    Private Sub Seconds()
        Sep2 = InStr(1, tbCallLength.Text, ":")
        Sep2 = Sep2 + 1
        varSeconds = CType(tbCallLength.Text.Substring(Sep2, 2), Double)
    End Sub

    Private Sub Pounds()
        Sep3 = InStr(2, tbCallPrice.Text, ".")
        Sep3 = Sep3 - 1
        varPounds = CType(tbCallPrice.Text.Substring(2, Sep3), Double)
    End Sub

    Private Sub Pence()
        Sep4 = InStr(2, tbCallPrice.Text, ".")
        Sep4 = Sep4 + 1
        varPence = CType(tbCallPrice.Text.Substring(Sep4, 2), Double)
    End Sub
End Class