Reference Master control from client page

Question

I'm trying to reference a Literal control inside a ContentPlaceHolder on my Master page from a client page using the following code:

    Private Sub TestMsgBox()
    Dim mpContentPlaceHolder As ContentPlaceHolder
    Dim mpLiteral As Literal

    mpContentPlaceHolder = CType(Master.FindControl("MainContent"), ContentPlaceHolder)
    If Not mpContentPlaceHolder Is Nothing Then
        mpLiteral = CType(mpContentPlaceHolder.FindControl("Literal1"), Literal)
        If Not mpLiteral Is Nothing Then
            mpLiteral.Text = "Some Text"
        End If
    End If
End Sub

This is from my Master page:

    <div id="div1" class="main" runat="server">
        <asp:ContentPlaceHolder ID="MainContent" runat="server">

            <asp:Literal ID="Literal1" runat="server"></asp:Literal>
        </asp:ContentPlaceHolder>           
    </div>

The code finds the MainContent control fine, but the Literal control keeps coming back as Nothing. Any idea what I'm doing wrong? This seems like it should be easy but apparently I'm wrong.

Thanks, Gina