Seen recently spiking in fatalmonitor
```
[Exception ErrorException] (/srv/mediawiki/php-1.31.0-wmf.2/extensions/Collection/includes/DataProvider.php:152) PHP Warning: Invalid argument supplied for foreach()
#0 /srv/mediawiki/php-1.31.0-wmf.2/extensions/Collection/includes/DataProvider.php(152): MWExceptionHandler::handleError(integer, string, string, integer, array, array)
#1 /srv/mediawiki/php-1.31.0-wmf.2/extensions/Collection/includes/BookRenderingMediator.php(70): MediaWiki\Extensions\Collection\DataProvider->fetchMetadata(array)
#2 /srv/mediawiki/php-1.31.0-wmf.2/extensions/Collection/includes/BookRenderingMediator.php(102): MediaWiki\Extensions\Collection\BookRenderingMediator->getBook(array)
#3 /srv/mediawiki/php-1.31.0-wmf.2/extensions/Collection/SpecialRenderBook.php(53): MediaWiki\Extensions\Collection\BookRenderingMediator->getBookFromCache(array)
```