To reproduce:

1. Have a loaded album with attached files
2. Drag and drop the album (not the files or tracks) onto the "Clusters" item on the left

Expected result: The files get added as a cluster

Actual result: Picard crashes with the following exception

Traceback (most recent call last):
  File "/home/phw/devel/musicbrainz/picard/./picard/tagger.py", line 527, in move_files
    self.cluster(files)
  File "/home/phw/devel/musicbrainz/picard/./picard/tagger.py", line 909, in cluster
    if len(objs) <= 1 or self.unclustered_files in objs:
TypeError: object of type 'generator' has no len()