I've had a look at the code and it's very lengthy and complex. Clearly, an object hasn't been built properly for some reason. So, I'm a bit stuck.
I couldn't find a bug report for this. I would probably enquire if this is actually causing something not to work? Can you live with it?
Also, make sure that you (at least) have the latest version of 4.1 (there have been loads of point releases) in case whatever it is has been fixed. There are definitelty changes in the file in the latest version (reading path is no longer on the line in your warning).