gridview pagesize value set by web.config not working

I would like to set the gridview pagesize value from a web.config entry.
Web.config has this:

<appSettings>
    <add key="pagingSize" value="20" />
</appSettings>

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gridview has this:

                <asp:GridView ID="gvCustomers1" runat="server" DataSourceID="odsCustomers" 
                    AutoGenerateColumns="False" AllowSorting="True" AllowPaging="True" 
                    EmptyDataText="No data exists" CssClass="siteTable"  
                    DataKeyNames="CustomerID" ViewStateMode="Enabled" 
                    PageSize="<%$  appSettings:pagingSize %>" >

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But this is not working.  I get "Int32Converter cannot convert from (null)".  If I hardcode it to 20, it works just fine.

Can you please tell me what I am doing wrong?  Thank you!