Sys.InvalidOperationException: Handler must be a function.

This exception comes when you give java script function name with parenthesis to an ajax control. e.g <%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="cc1" %>
...
....
<form id="form1" runat="server">
    <asp:ScriptManager ID="sm1" runat="server"></asp:ScriptManager>
    <cc1:TabContainer runat="server" ID="tbcTabContainer" OnClientActiveTabChanged="ChangeTab()"></cc1:TabContainer>
</form> Above OnClientActiveTabChanged="ChangeTab()" is not valid. Solution: Change OnClientActiveTabChanged="ChangeTab()" to OnClientActiveTabChanged="ChangeTab"