Assertion error when building in non-strict mode

After I did a change early in the dependency chain and didn't want to wait for the build to complete, I interrupted it and disabled strict mode. I got the following error. The element in question is a compose

[--:--:--][????????][build:base/freedesktop-sdk-junction.bst:desktop-image.bst] START   Caching Artifact
[00:12:11][????????][build:base/freedesktop-sdk-junction.bst:desktop-image.bst] BUG     Build

    An unhandled exception occured:
    
    Traceback (most recent call last):
      File "/home/abderrahim/dev/buildstream/buildstream/_scheduler/job.py", line 265, in child_action
        result = self.action(element)
      File "/home/abderrahim/dev/buildstream/buildstream/_scheduler/buildqueue.py", line 34, in process
        element._assemble()
      File "/home/abderrahim/dev/buildstream/buildstream/element.py", line 1149, in _assemble
        self.__artifacts.commit(self, assembledir, self.__get_cache_keys_for_commit())
      File "/home/abderrahim/dev/buildstream/buildstream/_artifactcache/ostreecache.py", line 265, in commit
        refs = [buildref(element, key) for key in keys]
      File "/home/abderrahim/dev/buildstream/buildstream/_artifactcache/ostreecache.py", line 265, in <listcomp>
        refs = [buildref(element, key) for key in keys]
      File "/home/abderrahim/dev/buildstream/buildstream/_artifactcache/ostreecache.py", line 47, in buildref
        assert key is not None
    AssertionError