Error message-Method not found

I get this error everytime I upload a file. 
" An error occured: 
Method not found: 'Void System.Web.HttpWorkerRequest.SendCalculatedContentLength(Int64)'.

 I hit Ok and the error goes away and the file is in the folder and seems to be OK. How can I stop this from coming up .

Please apply Microsoft .NET Framework 2.0 Service Pack 1 to get rid of the error.

In v2.9, we have rewritten the upload module for better performance thus it requires SP1.