I got this error message after upgrading my ASP.NET Dynamic Data site to ASP.NET 4.5. The exception was thrown each time an entity with a data annotation referencing a field or filter control was shown. I searched for days to find out what was wrong, including numerous references to replace the ScriptManager control with the ToolkitScriptManager, but that just wasn’t the issue. So, what I finally did was to create a new blank ASP.NET Web Application in Visual Studio 2012. I wanted it somewhat empty, so I could the bits and pieces from the Ajax Control Toolkit sample site, and work out when things went wrong. At first it just didn’t. I had a bare bone application with just the AutoComplete.aspx and the DefaultMaster.master files, as well as the Ajax Control Toolkit installed from NuGet. Worked just fine. Then I started adding the script related bits from my Dynamic Data project, but to no avail. Finally, I went over the Bin folders for a comparison between the two, and after listing the ones that were “extra” in the Dynamic Data project, I double-checked if they were all needed. Turns out the Microsoft.ScriptManager.MSAjax.dll assembly was no longer used by the web application project, so I deleted it. Damned, that did the trick; unbelievable.