Python libtorrent create torrent for empty file

create torrent successful for empty file in libtorrent == 1.1.8.0 but failed in libtorrent == 2.0.9.0

import libtorrent as lt 
fs = lt.file_storage()

lt.add_files(fs, file)

t = lt.create_torrent(fs, trunk_size)

# for track_url in track_urls:
#    t.add_tracker(track_url, 0)

t.set_creator('libtorrent %s' % lt.version)

for i in xrange(len(piece_hashes)):
    t.set_hash(i, piece_hashes[i])

# make torrent file
with open(torrent_file, 'wb+') as f:
    f.write(lt.bencode(t.generate()))

when file is empty, never to set_hash for t, and the code will error at t.generate() with missing or invalid ‘hash’ entry [libtorrent:175]