This is an older question, but a top hit on Google. So here is my answer. I have found this occurs due to some version conflict or early bug in Visual Studio and the Web Essential 2012 Extension. With an RTM install of Visual Studio 2012 and the latest Web Essentials (version 3.2 at the time of this post) I can reproduce this error.
Installing the latest update to Visual Studio has fixed the problem for me. Update
To confirm what version of Visual Studio you are running, goto Help -> About
Update 3 or greater should fix the issue
Microsoft Visual Studio Ultimate 2012
Version 11.0.60610.01 Update 3
Here is a link to the Visual Studio update on the Microsoft site