web.GetFolder() - value does not fall within the expected range. Sharepoint 2010

Below is the code to check if folder exist in doc library on a subsite and if not than create it.

using (SPSite site = new SPSite(sharePointSite))//"http://testsite.dev"))
                        {
                            using (SPWeb web = site.OpenWeb("/bdox"))
                            {
                                // Get doc library
                                SPList documentLibrary = web.Lists[documentLibraryName];
                                CopyFolder(parentfolder, documentLibrary, web);
                            }
                        }

CopyFolder(parentfolder, documentLibrary, web)
{
      string SharepointfolderURL = "/Shared Documents/TestFolder/";
      SPFolder newFolder = web.GetFolder(SharepointfolderURL);
      if (!newFolder.Exists)
      {
          web.Folders.Add(SharepointfolderURL);
          // Save changes
          web.Update();
      }

}

I am getting the an error on SPFolder newFolder = web.GetFolder(SharepointfolderURL); as value does not fall within the expected range.

The code works fine for RootWeb please help.