How do I spawn a window without a name?

I've tried:

session_name: default
windows:
 - windown_name:
 - pane:
 -

Which gives an str expected error.

session_name: default
windows:
 - windown_name: ''
 - pane:
 -

Which results in a blank window name.

I just want to do what I'd normally do in tmux with new-window.