I was able to make a test that reproduces your code, and expectedly fails. Also implemented a fix for it. See a temporary diff here: https://pastebin.com/C9JWkg0i

However, there is also a specific MS Windows version of _execute_child() (a phrase only computer-folks can say out loud without feeling very...wrong...). It uses a different method to extract and parse arguments, and it should be corrected and tested under windows, before I submit a PR for this.

Also, my test messes up the formatting. Instead of saying "....ok", they both say "... total 0\nok". I have no idea why.