I should add that I do have a functioning (but unpublished) page with this URL structure, but it was a bit tricky - largely because of this issue.
I had to jump through hoops just to convert the “year” and “month” path params to integers to feed to a “date”. I used hidden inputs as an “intermediary” to do the conversion (when there was otherwise no need for UI elements). It would have been much nicer to simply grab the “2019” path param (which is a string) and “convert to integer”.