Broken pull requests for API

While exploring the history of one of our projects via the API, I came across one of our earlier pull requests.

This is a request for the commits involved in a (merged) PR:

https://api.bitbucket.org/2.0/repositories/fairfax/coreplatforms-spock/pullrequests/17/commits

...but it yields a 404, with the error message:

{"error": {"message": "The source or destination could not be found."}}

This is consistent with the UI, which also reports no commits, but raises a bunch of questions:

1. How did we end up with a merged PR with no commits in it? Did we rebase and destroy all the hashes involved?
2. Surely an internal error like this should be a 500, not a 404?
3. It looks like the code naively falls back from displaying the commits to generating a list based on the source and destination branches - but those obviously aren't present in a merged PR.