Programatically add FilteredTextBoxExtender

I'm trying to programatically add a FilteredTextBoxExtender in Page_Load. I'm using the code here:

 
AjaxControlToolkit.FilteredTextBoxExtender accountIDSearchRowExtender = new AjaxControlToolkit.FilteredTextBoxExtender();
accountIDSearchRowExtender.TargetControlID = test.ClientID;

accountIDSearchRowExtender.FilterType = AjaxControlToolkit.FilterTypes.Numbers;

 

I am getting a valid ClientID for "test". I'm not getting an ...