Cannot connect to Prosody

[mcepl]

$ ./sleekmigrate.py -d -v -s ceplovi.cz -f users.csv 
INFO     Loading user file: users.csv
WARNING  The import method isn't unicode-safe, yet
Traceback (most recent call last):
  File "./sleekmigrate.py", line 355, in <module>
    extractor = XMPPAccountExtractor(auth['jid'], auth['pass'], plugin_config=plugin_config, plugin_whitelist=[])
  File "./sleekmigrate.py", line 140, in __init__
    sleekxmpp.ClientXMPP.__init__(self, jid, password, ssl, plugin_config, plugin_whitelist)
  File "/home/matej/.local/lib/python2.7/site-packages/sleekxmpp/clientxmpp.py", line 113, in __init__
    self.register_plugin('feature_starttls')
  File "/home/matej/.local/lib/python2.7/site-packages/sleekxmpp/basexmpp.py", line 261, in register_plugin
    pconfig = self.plugin_config.get(plugin, {})
AttributeError: 'bool' object has no attribute 'get'
$

I guess I am missing something, but I have SleekXMPP properly installed (via pip) and I really cannot find, why I deserve this crash.