AssertionError when caching artifact

bst 1.1.2 crashes whenever it tries to build glib-networking. Note that I have an open workspace for it:

[--:--:--][????????][build:core/glib-networking.bst      ] START   Caching Artifact
[00:00:29][????????][build:core/glib-networking.bst      ] BUG     Build

    An unhandled exception occured:
    
    Traceback (most recent call last):
      File "/home/mcatanzaro/.local/lib/python3.6/site-packages/buildstream/_scheduler/job.py", line 265, in child_action
        result = self.action(element)
      File "/home/mcatanzaro/.local/lib/python3.6/site-packages/buildstream/_scheduler/buildqueue.py", line 34, in process
        element._assemble()
      File "/home/mcatanzaro/.local/lib/python3.6/site-packages/buildstream/element.py", line 1145, in _assemble
        self.__artifacts.commit(self, assembledir, self.__get_cache_keys_for_commit())
      File "/home/mcatanzaro/.local/lib/python3.6/site-packages/buildstream/_artifactcache/ostreecache.py", line 265, in commit
        refs = [buildref(element, key) for key in keys]
      File "/home/mcatanzaro/.local/lib/python3.6/site-packages/buildstream/_artifactcache/ostreecache.py", line 265, in <listcomp>
        refs = [buildref(element, key) for key in keys]
      File "/home/mcatanzaro/.local/lib/python3.6/site-packages/buildstream/_artifactcache/ostreecache.py", line 47, in buildref
        assert key is not None
    AssertionError