Sending emails with the body based on RenderControl is generating errors

A few weeks back I posted looking for a way to send part of a html table in the body of an email from code behind.
The solution that works and was implemented was this logic:
StringBuilder sb = new StringBuilder(); StringWriter sw = new StringWriter(sb); HtmlTextWriter hw = new HtmlTextWriter(sw); gvDetails.RenderControl(hw); string html = sb.ToString(); But we recently added a ajax extender "AutoCompleteExtender" to the form/html table which is part of the above render logic, and now we are getting an error when the users submit the page and the email logic is executed.
We are getting the following error on the gvDetails.RenderControl(hw);
Extender control is not a registered extender control. Extender controls must be registered using RegisterExtenderControl() before calling RegisterScriptDescriptors(). Parameter name: extenderControl
Is the ...