Microsoft JScript runtime error: Sys.ArgumentUndefinedException: Value cannot be undefined. AJAX Editor

I am trying to load the AJAX Editor from "AjaxControlToolKit.7.1005" package on a 4.0 webform. Can anyone help? This is the error
"Microsoft JScript runtime error: Sys.ArgumentUndefinedException: Value cannot be undefined."
Here is the code:
<ajaxToolkit:ToolkitScriptManager ID="ToolkitScriptManager1" runat="server"></ajaxToolkit:ToolkitScriptManager> <cc1:Editor ID="Editor1" runat="server" Width="500px" Height="300px" />