E.g. https://en.wikibooks.org/wiki/a& -
```2015-05-20 19:22:07 mw1049 enwikibooks exception INFO: [5224434d] /wiki/a& InvalidArgumentException from line 240 of /srv/mediawiki/php-1.26wmf6/includes/Message.php: $key must be a string or an array
#0 /srv/mediawiki/php-1.26wmf6/includes/GlobalFunctions.php(1433): Message->__construct()
#1 /srv/mediawiki/php-1.26wmf6/includes/title/MalformedTitleException.php(44): wfMessage()
#2 /srv/mediawiki/php-1.26wmf6/includes/MediaWiki.php(117): MalformedTitleException->__construct()
#3 /srv/mediawiki/php-1.26wmf6/includes/MediaWiki.php(130): MediaWiki->parseTitle()
#4 /srv/mediawiki/php-1.26wmf6/includes/MediaWiki.php(518): MediaWiki->getTitle()
#5 /srv/mediawiki/php-1.26wmf6/includes/MediaWiki.php(431): MediaWiki->main()
#6 /srv/mediawiki/php-1.26wmf6/index.php(46): MediaWiki->run()
#7 /srv/mediawiki/w/index.php(3): include()
#8 {main} {"private":false}```
Seems to only be on non-wikipedias, so wmf6 regression?