Using ASP.NET AJAX continues to throw the "The Controls collection cannot be modified because the control contains code blocks" error

Hello,

I am using ASP.NET AJAX Control Toolkit with my .Net 4.0 ASP.NET application.  I did everything that was mentioned concerning the installation (downloaded the 4.0 binary ZIP file (v 4.1.7.0123), unblocked it, uncompressed it, and used the AjaxControlToolkit.dll as a project resource and used it to create a new area in my toolbox).  I then dragged a tab control to my page.  The page now has

<%@RegisterAssembly="AjaxControlToolkit"Namespace="AjaxControlToolkit"TagPrefix="act" %>

listed near the top of the page and within my body I am defining the tab control simply as

<act:TabContainer ID="TabContainer1" runat="server"> 

    <act:TabPanel ID="TabPanel1" runat="server"   HeaderText="Red Flower">  

            <ContentTemplate> Tab 1 Content  </ContentTemplate> 

   </act:TabPanel> 

</act:TabContainer>

When I try to run my app I get the error "The Controls collection cannot be modified because the control contains code blocks".  I know it is referring to ajax control toolkit becuase it I delete the TabContainer/TabPanel, I don't get the error.  I tried adding another Ajax Control Toolkit item but I got the same error.  Does anyone know why I am getting this error?  I myself am not defining any <% %> code blocks so I have to believe it is in the toolkit API. 

UPDATE: I am running this page as a web form within a master page.  The script manager is in the master.  If I just create a stand-alone page where I register the assembly and add the script manager, it appears to work.  But if I have my page within a master page, it doesn't.

Any clues would be greatly appreciated - Peter