api.err.UnknownStation when listing device

[cpiekacz]

When listing a client with $unifi_connection->list_clients($macaddr); I get the following error:

PHP Fatal error:  Uncaught Exception: Error message: api.err.UnknownStation in unifi-api/Client.php:4251
Stack trace:
#0 unifi-api/Client.php(1092): UniFi_API\Client->fetch_results()
#1 unifi-api/device.php(51): UniFi_API\Client->list_clients()
#2 {main}
  thrown in /root/unifi-api/Client.php on line 4251

Blocking and unblocking the same client works flawlessly.

UniFi OS Server version I'm quering is 4.3.6.