rsync error: error in rsync protocol data stream (code 12)

The internet failed to tell me:

This error can result from not having one of the directories present. Yes, I know that rsync creates lots of directories very nicely. Not all of them.

To wit:

rsync someDirectory someUser@

Gave the the data stream error until I created the directory 'system'.

I can't imagine why. The only thing that is distinctive about 'system' is that it is also ~/system, ie, at the top of the user's directory.