AJAX :: Navigate Between Tabs Using An Image Button?

I'm trying to navigate between tabs using an Image Button On each tab there is a button that navigates the user to te next tab. On the first tab, it works as expect from Tab 1 -> Tab2 But on the seconf tab, I get and error Java script

function MoveTab(tn) {
var container = $find('<%=
TabContainer1.ClientID %>').set_activeTabIndex(tn); ;
}

Button first tab that works ok

<asp:ImageButton
ID="ImageButton8"
runat="server"
OnClientClick="javascript:MoveTab('1')"
ImageUrl="~/Images/saveadd.gif"
/>

Button on second tab that gets error message

<asp:ImageButton
ID="ImageButton1"
runat="server"
OnClientClick="javascript:MoveTab('2')"
ImageUrl="~/Images/savereview.gif"
/>

Error Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. Exception Details: System.InvalidCastException: Specified cast is not valid. Source Error:

[Code]....

Stack Trace:

[Code]....