Error attempting to delete project group

When trying to delete a project group:

javax.jdo.JDOUserException: One or more instances could not be deleted NestedThrowables: javax.jdo.JDODataStoreException: Delete request failed: DELETE FROM PROJECTDEPENDENCY WHERE PROJECTDEPENDENCY_ID=? NestedThrowables: SQL Exception: DELETE on table 'PROJECTDEPENDENCY' caused a violation of foreign key constraint 'PROJECT_FK2' for key (5107). The statement has been rolled back.

A snippet of the stack trace:

javax.jdo.JDOUserException: One or more instances could not be deleted
at org.jpox.AbstractPersistenceManager.deletePersistentAll(AbstractPersistenceManager.java:1525)
at org.jpox.AbstractPersistenceManager.deletePersistentAll(AbstractPersistenceManager.java:1498)
at org.jpox.store.rdbms.scostore.FKListStore.clear(FKListStore.java:1137)
at org.jpox.store.mapping.CollectionMapping.deleteDependent(CollectionMapping.java:334)
at org.jpox.store.rdbms.table.ClassTable.deleteDependent(ClassTable.java:2651)
at org.jpox.store.StoreManager.deleteDependent(StoreManager.java:1019)
at org.jpox.state.StateManagerImpl.deletePersistent(StateManagerImpl.java:4405)
at org.jpox.AbstractPersistenceManager.internalDeletePersistent(AbstractPersistenceManager.java:1473)
at org.jpox.AbstractPersistenceManager.deletePersistent(AbstractPersistenceManager.java:1489)
at org.codehaus.plexus.jdo.PlexusJdoUtils.removeObject(PlexusJdoUtils.java:121)
at org.apache.maven.continuum.store.JdoContinuumStore.removeObject(JdoContinuumStore.java:1906)
at org.apache.maven.continuum.store.JdoContinuumStore.removeBuildResult(JdoContinuumStore.java:600)
at org.apache.maven.continuum.DefaultContinuum.removeProject(DefaultContinuum.java:719)
at org.apache.maven.continuum.DefaultContinuum.removeProjectGroup(DefaultContinuum.java:278)
at org.apache.maven.continuum.web.action.ProjectGroupAction.remove(ProjectGroupAction.java:263)
[...]