popup control extender - problem with master page!

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C#

ASP.NET4

Hi,

I made 2 pages, one with masterpage and one without master page. the one without masterpage is working fine when adding the popup control extender inside the gridview after clicking a button. on the other hand, the page with the masterpage it brings an error:

The Controls collection cannot be modified because the control
contains code blocks (i.e. <% ... %>).
so I read somewhere that i need to add some stuff to the web.config but i am sure if what I did is correct!:

<compilation debug="true" targetFramework="4.0">

   <assemblies>
     <add assembly="AjaxControlToolkit"/>
   </assemblies>

</compilation>

<pages>

    <controls>

      <add assembly="AjaxControlToolkit" namespace="AjaxControlToolkit" tagPrefix="cc1" />    
                
    </controls>
            
</pages>

I am doing something wrong or am i missing something?

Thanks.