How to assign a default value to a textbox insert template. Thanks

I want the user to first enter a value into a textbox insert template before selecting a value from a dropdownlist, in order to avoid a

InvalidCastException. I needed help handling this exception. Maybe assigning a default 1 will help?? Please see the code below

Protected Sub DropDownList5_SelectedIndexChanged(ByVal sender As Object, ByVal e As EventArgs)
        Dim ddl As DropDownList = TryCast(sender, DropDownList)
        Dim row As DetailsView = TryCast(DetailsView1, DetailsView)
        Dim tbox1 As TextBox = TryCast(row.FindControl("TextBox1"), TextBox)
        Dim tbox2 As TextBox = TryCast(row.FindControl("TextBox2"), TextBox)
        Dim tbox3 As TextBox = TryCast(row.FindControl("TextBox3"), TextBox)
        Dim tbox4 As TextBox = TryCast(row.FindControl("TextBox4"), TextBox)

        Try
            If tbox4 IsNot Nothing Then
                tbox2.Text = ddl.SelectedItem.Text
                tbox3.Text = ddl.SelectedValue.ToString
                tbox1.Text = tbox4.Text * tbox3.Text
                TextBox5.Text = ddl.SelectedValue.ToString
            End If

        Catch ex As InvalidCastException
            Dim txt As String = "Enter Quantity Ordered First"
            MsgBox(txt)
        Catch ex As Exception
            Dim txt As String = "Exception"
            MsgBox(txt)
        End Try
    End Sub