what is the Request URL generated on that page when you click the Try It button?
i don’t see any issues:
- https://api.inaturalist.org/v1/identifications/categories?user_id=pisum
- https://api.inaturalist.org/v1/identifications/categories?user_id=tiwane
- https://api.inaturalist.org/v1/identifications/categories?user_id=pisum,tiwane
…
not sure what you’re trying to do, but this page may or may not be useful: https://forum.inaturalist.org/t/many-ids-have-id-category-null/19061/4