Extender control ValidatorCalloutExtender cannot extend TextBox

Extender control 'ValidatorCalloutExtender1' cannot extend 'TextBox1'. Extender controls of type 'AjaxControlToolkit.ValidatorCalloutExtender' cannot extend controls of type 'System.Web.UI.WebControls.TextBox'.

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.InvalidOperationException: Extender control 'ValidatorCalloutExtender1' cannot extend 'TextBox1'. Extender controls of type 'AjaxControlToolkit.ValidatorCalloutExtender' cannot extend controls of type 'System.Web.UI.WebControls.TextBox'.

Solution: Make sure the TargetControlID of the ValidatorCalloutExtender is set to a validator control instead of the TextBox control

Example:

<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>

   <asp:RequiredFieldValidator ID="RequiredFieldValidator1" runat="server" 
        ErrorMessage="Required field" Display="None" 
        ControlToValidate="TextBox1" />
        
   <cc1:ValidatorCalloutExtender ID="ValidatorCalloutExtender1" 
        TargetControlID="RequiredFieldValidator1" runat="server" />