ComboBox.Items.Count is equal 0 after databinding of its Store

[AlexMaslakov]

protected void Page_Load(object sender, EventArgs e)
 {
  if (!Ext.Net.ExtNet.IsAjaxRequest)
    {
             ServicesCmbStore.DataSource = GetServices().Select(x => new { Key = x.Id, Value = x.Name });
             ServicesCmbStore.DataBind();
             ServicesCmb.SelectedItem.Value = myTuple.Item1.ToString();
             //ServicesCmb.Items.Count == 0 ?????????
    }
}



<ext:ComboBox ID="ServicesCmb" runat="server" ValueField="Key" DisplayField="Value">
                                    <Store>
                                        <ext:Store ID="ServicesCmbStore" runat="server">
                                            <Reader>
                                                <ext:JsonReader IDProperty="Key">
                                                    <Fields>
                                                        <ext:RecordField Name="Key" />
                                                        <ext:RecordField Name="Value" />
                                                    </Fields>
                                                </ext:JsonReader>
                                            </Reader>
                                        </ext:Store>
                                    </Store>
 </ext:ComboBox>

Why is ComboBox.Items.Count equal zero?