calendar Extender with textbox readonly

<asp:TextBox ID="txt_due_date" runat="server" ReadOnly="true"></asp:TextBox>
<asp:ImageButton ID="img_calendar" runat="server" CausesValidation="False" ImageUrl="app_images/Calendar.png" />
<cc1:CalendarExtender ID="img_calendar_CalendarExtender" runat="server" TargetControlID="txt_due_date"
Format="MMMM d, yyyy" PopupButtonID="img_calendar">
                                                                        </cc1:CalendarExtender>

the code works, but when i retrieve the value in code behind, its always null