get_selected_streams dosen't generate root_metadata

[tkirschke]

I set up everything to use tap-mailchimp and created a valid config file. The untouched catalog I get after running the discovery mode looks good (so far). After selecting the streams I want to extract and running the tap witch --catalog nothing happens. No message or whatsoever appears in the console.
I dived into the tap-mailchimp sourcecode and landed at the get_selected_streams_function in sync.py. I printed out the metadata (mdata) which looks fine. But in the next step, where root_metadata = mdata.get(()) gets defined, there seems to be something broken.
I also printed out the root_metadata and its always "None". That even happens with an untouched catalo, fresh out of the discovery mode. Althoug at this step there is no check, if selected = true.
Can anybody explain to me whats wrong? And maybe what .get(()) is supposed to do?
I appreciate any help, thanks in advance!