Theme downloads from Features Servers broken

The (apparently standard) way of downloading a theme from a feature server appears to be broken, and returns this error:

Undefined property: DrushMakeProject_Theme::$download drush_make.project.inc:25

Also, this is only visible using --debug, as it comes out as a notice. The folder is created, but ends up empty.

This is based on the current Open Atrium makefile at lines 183-4 of http://drupalcode.org/project/openatrium.git/blob/HEAD:/openatrium.make:

projects[ginkgo][location] = "http://featureserver.phase2technology.com/fserver"
projects[ginkgo][version] = "1.0-alpha10"

Note that the tarball exists, as this works:

projects[ginkgo][download][type] = "file"
projects[ginkgo][download][url] = "http://featureserver.phase2technology.com/sites/default/files/fserver/ginkgo-6.x-1.0-alpha10.tgz"

Any ideas?