Button event not fired inside of user control when validation is enabled.

Hello,

I have used a user control in a Tabpanel of Tabconatiner as below:

<ajaxtoolkit:TabContainer ID="TabContainer1" runat="server" ActiveTabIndex="0"  AutoPostBack="true" OnActiveTabChanged="Tabs_ActiveTabChanged">
        <ajaxtoolkit:TabPanel runat="server" HeaderText="Week-2" ID="TabPanel1"  >
            <ContentTemplate>
                 <asp:UpdatePanel ID="UpdatePanel1" runat="server" UpdateMode="Conditional">
                 <ContentTemplate>
                    <uc1:Assessment_Input ID ="InputContainer1" runat="server" /> 
                 </ContentTemplate>
                 </asp:UpdatePanel>          
            </ContentTemplate>
        </ajaxtoolkit:TabPanel>
........

The usercontrol inturn has a tabcontainer with a TabPanel like below:

....
<ajaxtoolkit:TabPanel ID="TabPanel2" runat="server" HeaderText="Tab2">
        <ContentTemplate>
            X : <asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
            <asp:RequiredFieldValidator ID="RequiredFieldValidator1" runat="server" ControlToValidate="TextBox1"
           CssClass="failureNotification" ErrorMessage="Please Select Value"
           ValidationGroup="tab2"></asp:RequiredFieldValidator>
            y: <asp:TextBox ID="TextBox2" runat="server"></asp:TextBox>
            <asp:RequiredFieldValidator ID="RequiredFieldValidator2" runat="server" ControlToValidate="TextBox2"
           CssClass="failureNotification" ErrorMessage="Please Select Value"
           ValidationGroup="tab2"></asp:RequiredFieldValidator>
            <asp:Button ID="Button1" runat="server" Text="Button" OnClick="Button1_Click" ValidationGroup="tab2" />
        </ContentTemplate>
    </ajaxtoolkit:TabPanel>

...

Button1_click doesn't get fired when validationgroup is set, but if I disable by adding Causesvalidation=false, the event gets fired.

Any inputs on this will be really helpful.