We get log backups from the secondary node. But now I see that log_since_last_log_backup_mb in sys.dm_db_log_stats returns null for databases which are in Availability groups, so Ola Hallengren's log backup procedure with 

    @LogSizeSinceLastLogBackup = 25,@TimeSinceLastLogBackup = 21000 

limitations works on those databases. The backup process is currently working on the secondary node. 

But I could not understand why it returns null? Have you encountered such a situation? 

The following pictures show result of log size since last log backup on the primary and secondary nodes.

**Primary Node**

[![enter image description here][1]][1]


**Secondary Node**

[![enter image description here][2]][2]


  [1]: https://i.sstatic.net/qWzdQ.jpg
  [2]: https://i.sstatic.net/hC5up.jpg