Comment 6 for bug 394097

The root cause is messages in the Bug.indexed_messages collection that point to a parent message that is not in the collection. The API machinery tries to calculate a canonical URL for the message, but cannot do so because it has not been wrapped in the IndexedMessage delegate. The API machinery seems to cope fine when the parent is a member of the bug messages collection; presumably there's a cache or something similar at work there.

Now that we know the root cause, it's apparent that this bug is a duplicate of bug #279561. However, as a branch has been linked here, and activity is happening around this bug, I have marked the older bug a duplicate of this one.