preseed_fetch no longer works with relative URL

My installer preseed file contains the following line, which worked in natty:

d-i preseed/late_command string preseed_fetch post-install.sh /tmp/post-install.sh && sh /tmp/post-install.sh > /var/log/post-install.out 2>&1

In oneiric, it fails with this rather cryptic error:

/bin/fetch-url: .: line 33: can't open /usr/lib/fetch-url//post-install.sh

It appears these scripts have been reworked for oneiric to be more modular in their protocol support:

http://changelogs.ubuntu.com/changelogs/pool/main/p/preseed/preseed_1.49ubuntu1/changelog

However, the backward compatibility support seems to be broken for certain cases. make_absolute_url in /lib/preseed/preseed.sh is apparently failing to make the URL absolute. I notice it uses the last location to try to resolve relative URLs. However, this is my only usage of fetch, and /var/run/preseed.last_location does not exist.

Making the URL absolute causes the fetch to succeed.