If you have recently installed a package with T4Scaffolding dependency (ex. MVCMailer uses T4Scaffolding.Core),
then you can uninstall T4Scaffolding.Core and restart VS 2013. Notice that MvcMailer which caused this in my case, won’t work in 2013. Best is to check your references or packages for suspects.
From comments:
Uninstalling it didn’t seem to work for me, so I deleted packages/T4Scaffolding from the disk* and then it worked. (by Jared Thirsk)