'System.Web.UI.Page' does not contain a definition for 'DataItem'

Question

<span class="telerik_bx">
    <telerik:RadComboBox runat="server" 
                         ID="LowCategoryCombobox" 
                         EmptyMessage="---선택---" 
                         DataSourceID="LowCategory"  
                         DataTextField="CategoryName"
                         SelectedValue='<%# DataBinder.Eval(Container.DataItem, "LowCategoryIdx") %>' 
                         DataValueField="LowCategoryIdx" 
                         Skin="Windows7"  >
    </telerik:RadComboBox>
</span>

System.Web.UI.Page' does not contain a definition for 'DataItem' and no extension method 'DataItem' accepting a first argument of type 'System.Web.UI.Page' could be found

There is a red line at DataItem

What is the problem?