As of 14:25UTC 2019-10-12, Mediawiki is logging a high rate (~2.7k/minute errors) of this:
ErrorException from line 596 of /srv/mediawiki/php-1.35.0-wmf.1/includes/api/ApiQueryBase.php: PHP Notice: Undefined property: stdClass::$page_namespace
Errors are almost entirely enwiki, and look to occur bucketed at 5-minute intervals.
https://logstash.wikimedia.org/goto/68696e23e000f401450e528e03fcdffe
Sample Logstash document: https://logstash.wikimedia.org/app/kibana#/doc/logstash-*/logstash-mediawiki-2019.10.12/mediawiki?id=AW3AnCXDx3rdj6D8OARR&_g=h@44136fa
```
#0 /srv/mediawiki/php-1.35.0-wmf.1/includes/api/ApiQueryBase.php(596): MWExceptionHandler::handleError(integer, string, string, integer, array)
#1 /srv/mediawiki/php-1.35.0-wmf.1/includes/api/ApiQueryBacklinksprop.php(289): ApiQueryBase->executeGenderCacheFromResultWrapper(Wikimedia\Rdbms\ResultWrapper, string)
#2 /srv/mediawiki/php-1.35.0-wmf.1/includes/api/ApiQueryBacklinksprop.php(83): ApiQueryBacklinksprop->run()
#3 /srv/mediawiki/php-1.35.0-wmf.1/includes/api/ApiQuery.php(255): ApiQueryBacklinksprop->execute()
#4 /srv/mediawiki/php-1.35.0-wmf.1/includes/api/ApiMain.php(1588): ApiQuery->execute()
#5 /srv/mediawiki/php-1.35.0-wmf.1/includes/api/ApiMain.php(537): ApiMain->executeAction()
#6 /srv/mediawiki/php-1.35.0-wmf.1/includes/api/ApiMain.php(508): ApiMain->executeActionWithErrorHandling()
#7 /srv/mediawiki/php-1.35.0-wmf.1/api.php(83): ApiMain->execute()
#8 /srv/mediawiki/w/api.php(3): require(string)
#9 {main}
```