SelectedIndexChanged event of Asp.Net CheckBoxList control is not firing due to PopupControlExtender

I have created a multiselect dropdown list control by using textbox,checkboxlist,panel and popupcontrolextender.I have placed all the controls inside UpdatePanel. Now when i checked a checkbox in the CheckBoxList then SelectedIndexChanged of CheckBoxList getting fired but when i unchecked it then its not getting fired. However when i tried after removing PopupControlExtender in my given code then this event is firing correctly. I think PopupControlExtender causing the problem. Please help me to solve this issue ?

Here is the code:

<asp:UpdatePanel ID="updatepanelBizUnit" runat="server"><ContentTemplate><asp:TextBox ID="txtBizUnit" runat="server" Width="147px" ReadOnly="true" TabIndex="1"></asp:TextBox><asp:RequiredFieldValidator ID="valRequiredBizUnit" CssClass="validatorMessage" Display="Dynamic"
            ValidationGroup="VgroupPart" runat="server" ErrorMessage="The 'Business Unit' field is required."
            Text="*" ControlToValidate="txtBizUnit" ForeColor="" SetFocusOnError="True"></asp:RequiredFieldValidator><asp:PopupControlExtender ID="txtBizUnit_PopupControlExtender" runat="server" Enabled="True"
            ExtenderControlID="" TargetControlID="txtBizUnit" PopupControlID="PnlBizUnit"
            OffsetY="14"></asp:PopupControlExtender><asp:Panel ID="PnlBizUnit" runat="server" Height="116px" Width="145px" BorderStyle="Solid"
            BorderWidth="1px" Direction="LeftToRight" ScrollBars="Auto" BackColor="#FFFFFF"
            Style="display: none"><asp:CheckBoxList ID="ChkBizUnit" runat="server" AutoPostBack="true" OnSelectedIndexChanged="ChkBizUnit_SelectedIndexChanged"></asp:CheckBoxList></asp:Panel></ContentTemplate><Triggers><asp:AsyncPostBackTrigger ControlID="ChkBizUnit" EventName="SelectedIndexChanged" /></Triggers></asp:UpdatePanel>