Expressions do not work in prepareBuildSubjectFormat

The subject line for the PREPARE BUILD notification in application.xml defaults to:

<prepareBuildSubjectFormat>[continuum] PREPARE BUILD ${state}: ${project.projectGroup.name}</prepareBuildSubjectFormat>

The ${state} expression is substituted correctly, but not ${project.projectGroup.name} (or any ${project.*} expressions I've tried.)

The continuum.log file shows:

2009-03-09 12:07:45,308 [pool-7-thread-1] INFO org.apache.maven.continuum.scm.queue.PrepareBuildProjectsTaskExecutor - Ending prepare build
2009-03-09 12:07:45,358 [pool-7-thread-1] WARN velocityComponent - org.apache.velocity.runtime.exception.ReferenceException: reference : template = subjectPattern [line 1,column 37] : ${project.projectGroup.name} is not a valid reference.
2009-03-09 12:07:45,378 [pool-7-thread-1] INFO org.apache.maven.continuum.notification.mail.MailContinuumNotifier - Message Subject: '[continuum] PREPARE BUILD ERROR: ${project.projectGroup.name}'.

The same kind of expressions do work correctly in the regular build notification:

2009-03-09 12:09:54,833 [pool-1-thread-2] INFO org.apache.maven.continuum.notification.mail.MailContinuumNotifier - Message Subject: '[continuum] BUILD SUCCESSFUL: Multi Module Example - Multi Module Example - Default Maven 2 Build Definition'.