by Brian King.
Good question.
It would be nice if the backup / restore developer documentation got a good reworking, now that things have been finalized.
Perhaps you're experiencing the same problem as this: http://tracker.moodle.org/browse/MDL-26442?focusedCommentId=107686&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-107686 ?
Eloy mentions a patch near the end. It didn't work for me, but perhaps it will for you.