Bind asp:RadioButton in datalist

Hello, I'm trying to bind data to asp:RadioButton in datalist <asp:RadioButton ID="rdCover" runat="server" Checked='<%# Eval("gallery_cover") %>' /> my value comes from SQL query: select CASE gallery_cover WHEN 0 THEN 'false' ELSE 'true' END AS xx from table But I get error "Specified cast is not valid". I'm using VS2008 thank