(OP here.)

Aargh! Of course I checked this carefully before I posted, so when I saw your reply (and stevieb's equivalent advice), I sniffed "Yes, I'm sure the path to the directory exists!"

Then I checked more carefully, and realized that I'd been passing in a relative path from my main directory that wasn't the same as the relative path from the test directory where I was actually running the script....

I'm sorry to have wasted everyone's time with something so obvious. I actually tried to rubber-duck it first! sigh I guess I'm glad my code was OK, I just didn't know how to run it :-(