asp.net ValidatorCalloutExtender unhandled exception error

Unhandled exception at line 79, column 1 in http://localhost:49456/about_us.aspx?_TSM_HiddenField_=ToolkitScriptManager_HiddenField&_TSM_CombinedScripts_=;;AjaxControlToolkit,+Version=1.0.20229.20821,+Culture=neutral,+PublicKeyToken=28f01b0e84b6d53e:en-US:c5c982cc-4942-4683-9b48-c2c58277700f:e2e86ef9:9ea3f0e2:9e8e87e9:1df13a87:4c9865be:ba594826:757f92c2
 
0x800a139e - JavaScript runtime error: Sys.ArgumentNullException: Value cannot be null.
 

above error occured for the following code
 
<tdstyle="text-align:left"class="content"><asp:TextBoxID="txtName"Width="225px"AutoPostBack="true"runat="server"ValidationGroup="valEnquiry"CausesValidation="True"TabIndex="1"></asp:TextBox><br/><asp:RequiredFieldValidatorID="RFName"runat="server"ValidationGroup="valEnquiry"ControlToValidate="txtName"ErrorMessage="Please Enter Your Name"ForeColor="Red"></asp:RequiredFieldValidator><cc1:ValidatorCalloutExtenderID="val_reqDTT"runat="server"TargetControlID="RFName"></cc1:ValidatorCalloutExtender></td>