Grails GoogleAppEngine no metadata problem

I am trying to run grails on google app engine using JDO, but I got this:

org.codehaus.groovy.runtime.InvokerInvocationException: javax.jdo.JDOFatalUserException: No meta data for Book. Perhaps you need to run the enhancer on this class? NestedThrowables: org.datanuc开发者_JAVA百科leus.exceptions.NucleusUserException: No meta data for Book. Perhaps you need to run the enhancer on this class?

I am confused, because when I did "grails app-engine package" it looks like the DataNucleus enhancer was running.

How can I fix this?

Thanks

Trivial fix, I just have to put the domain class into a package, i.e. grails create-domain-class packageName.Book