Visual C#

Recently I was working on conversion of a social network project from ASP.NET 3.5 to ASP.NET 4.0. Everything went pretty smoothly except for AjaxControlToolkit Asynchronous File Upload control. Basically the uploading of file did not work, on the staging server nothing happening during the file upload... but in debug mode the following error was thrown ((AjaxControlToolkit.AsyncFileUpload)(sender)).FileContent' threw an exception of type 'System.NullReferenceException' with status message "The file attached is invalid." 

It seemed pretty odd because the code was not modified and everything functioned on .NET 3.5. This is .NET version incompatibility with AjaxControlToolkit.