Created Issue: Custom tool error: Unable to load one or more of the requested types. [96]

I'm getting the following error in VS when I save my cshtml helpers. The generated file is deleted also.
```
Error 2 Custom tool error: Unable to load one or more of the requested types. Retrieve the LoaderExceptions property for more information.

```

I've recently moved to VS2012, and this is the first time I have edited my helpers since. Any ideas?