To prevent duplicates, the mass import API checks if the archivist has already bookmarked the external work currently being bookmarked. To do this, it filters the archivist's bookmarks on bookmarkable with a url field set to the external work's url. Unfortunately, if the bookmarkable is a work or series, it doesn't have a url field so this code crashes if the archivist has ever bookmarked anything that isn't an external work.
This is covered by an automated test but can be tested manually from a REST client by sending the following POST request (this format can be pasted into RubyMine, or adapted to a REST client - token for Test will be provided on request):
Right now, this test will return the following response:
But after the fix the expect response should be: