Incremental backup from a streaming replication standby

I played around with incremental backup yesterday and tried $subject

The WAL summarizer is running on the standby server, but when I try  
to take an incremental backup, I get an error that I understand to mean  
that WAL summarizing hasn't caught up yet.

I am not sure if that is working as designed, but if it is, I think it  
should be documented.

Yours,
Laurenz Albe