Nested MasterPages FindControl TextBox.

In the following code and am trying to retrieve the text from a textbox from a child masterpage. The code returns a null value, I'm unsure why this returns null because in debug mode I am able to see value entered.
 
Parent Master:
                <!-- Content Body Holder -->
                <div style="border-width: 1em; border-color: #000;">
                    <asp:ContentPlaceHolder id="defaultBodyCPH" runat="server"></asp:ContentPlaceHolder>
                </div>

Child Master:
<asp:Content ID="Content2" ContentPlaceHolderID="defaultHeaderCPH" Runat="Server">
                    <!--This section contains the Website Search functionality -->
                    <div style="float: right; background:#fff; border:solid .1em #000;">
                        <asp:Textbox id="searchTB" runat="server" width="20em" height="2.25em" cssClass="searchBox" OnTextChanged="Search" />
                        <asp:ImageButton id="cmdSearch" runat="server" ImageUrl="~/images/searchIcon.png" ImageAlign="Middle" style="margin-left: -.5em;" onclick="Search"/>
                        <asp:textboxwatermarkextender runat="server" ID="searchTBWE" TargetControlID="searchTB" WatermarkText="Enter Search Criteria" WatermarkCssClass="watermarked" />
                    </div>
</asp:Content>

Codebehind:
'This code works in conjunction with the Event handler in the ListViewMasterPage VB code
            Dim masterPage As MasterPages_ListViewMasterPage = TryCast(Me.Master, MasterPages_ListViewMasterPage)
            Dim cp As ContentPlaceHolder = CType(Me.Master.Master.FindControl("defaultBodyCPH"), ContentPlaceHolder)
            'Dim db As ClientInfoDataContext = New ClientInfoDataContext
            Dim search As String
            If Not IsNothing(masterPage) Then
                Dim searchTBx As TextBox = CType(cp.FindControl("searchTB"), TextBox)
                search = searchTBx.Text
            End If