asp.net SqlDataSource for GridView with Array parameter

I've got a Web User Control

<asp:GridView ID="gvPodInventory" runat="server" AutoGenerateColumns="True" DataSourceID="dsIseriesPodInventory" DataKeyNames="SKU"
                AllowSorting="True" CellPadding="4" ForeColor="#333333" GridLines="Vertical" Width="2000px"  >
</asp:GridView>

Which is used in my ASPX

<asp:SqlDataSource ID="dsIseriesPodInventory" runat="server" 
        ConnectionString="<%$ ConnectionStrings:VM520ConnectionString %>" 
        ProviderName="<%$ ConnectionStrings:VM520ConnectionString.ProviderName %>"
        CancelSelectOnNullParameter="False">
</asp:SqlDataSource>

Along with a textbox for the input filter

<asp:TextBox runat="server" ID="txtSKU" ></asp:TextBox>

In the ASPX.CS the following Page_Load code

dsIseriesPodInventory.SelectCommandType = SqlDataSourceCommandType.Text;
dsIseriesPodInventory.SelectCommand = " SELECT FFSKU SKU, FFDEPT DPT " + 
                                        "FROM KIVALIB.FFINVPF " +
                                        "WHERE FFSKU in (?)";


//Build Where Parm
ControlParameter cp = new ControlParameter("SKU", TypeCode.String, "txtSKU", "Text");
cp.DefaultValue = "0";

dsIseriesPodInventory.SelectParameters.Add(cp);

This works fine and dandy if i input a single integer value into the textbox. If i put in multiple comma separated values it throws a format exception.

When I alter the .SelectCommand to specify WHERE xx IN (1234,123,15554) then nothing is displayed in the grid... even though the values are valid and in the table.

The data in the table is numeric, and i've verified the query executes just fine.