After adding APS , Initial clean causes an error

Using Eclipse, I was just trying to augment an existing purely Java Android app by adding a simple Scala class that would provide some functionality.

1. I clicked on the project to "Add AndroidProguardScala Nature" .
2. Immediately afterwards, I wanted to issue a "clean" to make sure I was starting at a good state and I received this error

Errors occurred during the build.
Errors running builder 'Android Proguard Scala Builder' on project 'com.SerialTest.SerialTestActivity'.
java.lang.NullPointerException

I've tried this with several programs that I had written as well as some samples from the book Android Wireless Application Development Volume I

[banshee]

Did you add AndroidProguardScala without having Scala nature on it first? And if you did, did adding Scala nature make the problem go away?