How To Add List Items To Dropdownlist In A Formview With Vb.net

I have a dropdownlist in a formview that looks like this:

<asp:DropDownList ID="DropDownList5" runat="server" AppendDataBoundItems="True"
DataSourceID="SqlDataSource6" DataTextField="StaffName"
DataValueField="StaffName" SelectedValue='<%# Bind("LetterName") %>'
Width="155px">
<asp:ListItem Value=""> </asp:ListItem>
</asp:DropDownList>

I am trying to add the value from a textbox which is also in the formview into the dropdownlist as a list item. I currently have this code for it:
Protected Sub Formview1_PreRender(ByVal sender As Object, ByVal e As System.EventArgs) Handles FormView1.PreRender
Dim dropdown As DropDownList
Dim listvalue As TextBox
dropdown = FormView1.FindControl("dropdownlist5")
listvalue = FormView1.FindControl("textbox1")
dropdown.Items.Add(New ListItem(listvalue.Text, listvalue.Text))
End Sub

This code works and adds the new item to the dropdownlist if I have no SelectedValue set for my dropdownlist. However when I set the selectedValue I get the error message:

"'DropDownList5' has a SelectedValue which is invalid because it does not exist in the list of items.

Parameter name: value"

This is the bit I dont understand because the value does now exist in the list of items as it was added in from the textbox value. Does it try to do the selectedValue before adding my list item? If so how can I add the list item first, or do I need to set the selectedValue in the code after adding the list item?