Gradle-Grails-Plugin NoClassDefFoundError during "test" task while adding generated-sources

gradle 2.2 grails 2.3.8 grails-gradle-plugin 2.1.2

I have a custom protobuf plugin that generates a bunch of .java files into src/java/generated-sources. When i execute the “test” task from the grails-gradle-plugin i end up with a NoClassDefFoundError on one of my compiled protobufs. The compiled class exists in build/classes/. I can start up my app fine with no errors. I don’t understand why this class isn’t on the classpath when i run the “test” task.