AssertionError when caching artifact

This seems to be happening for every element in the build pipeline with bst version 1.1.3+4.g0916d81e

[--:--:--][????????][build:base/freedesktop-sdk-junction.bst:desktop/vulkan.bst] START   Caching artifact
[00:00:44][????????][build:base/freedesktop-sdk-junction.bst:desktop/vulkan.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 320, in _child_action
        result = self._action_cb(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 1497, 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 135, in commit
        refs = [self.get_artifact_fullname(element, key) for key in keys]
      File "/home/mcatanzaro/.local/lib/python3.6/site-packages/buildstream/_artifactcache/ostreecache.py", line 135, in <listcomp>
        refs = [self.get_artifact_fullname(element, key) for key in keys]
      File "/home/mcatanzaro/.local/lib/python3.6/site-packages/buildstream/_artifactcache/artifactcache.py", line 152, in get_artifact_fullname
        assert key is not None
    AssertionError