Why doesn't the photos.getPhotos API return photos if I add fields=photo.pic_max to the request?

Question

Classmates

Hello.

https://api.ok.ru/fb.do?application_key=***&count=10
&fields=photo.PIC_MAX&format=json
&method=photos.getPhotos&gid=***
&direction=FORWARD&aid=***&sig=***&access_token=***

I send a request like ^^, which
1. successfully returns an array with a photo if "&fields=photo.PIC_MAX" is NOT specified
2. returns an EMPTY array with a photo if "&fields=photo.PIC_MAX" is specified.
My goal is to get a link to the photo in original quality.
Question:
How to correctly specify fields=photo.PIC_MAX so that photos are returned?
Or maybe you can get links to photos in original quality somehow differently?
I will be grateful for any help.