The System.Windows.Interactivity namespace *vanishes* when I swap from DEBUG build to RELEASE build. My project fails to build (naturally), and gives the error and warning visible in the images below. The namespace simply disappears from the Object Browser as soon as I switch build modes, even though the reference is still present in the Solution Explorer.

What's going on here?! And how do I fix it?

[![Debug Mode](https://i.stack.imgur.com/Rg4Qi.png)](https://i.stack.imgur.com/Rg4Qi.png)

[![Release Mode](https://i.stack.imgur.com/IqUnt.png)](https://i.stack.imgur.com/IqUnt.png)