Scheduling in OSvC and Compressed CSV file

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We are using OSvC KA (but the same I encountered in KF) and I created the simple report. The results I wanted to be send via scheduling. Because the resulted data set is very long, I opted to use Compressed CSV file format of attached output. When I want to save the schedule, I get the warning message "The report must contain sort columns and the first sort column must be on a primary key ascending." The report is sorted by Answer ID which is (as I think) a primary key.

What else should I set? I didn't find anything in documentation or on web. Thank you for help or hint.