Rarely happening:
```
2017-10-19 15:59:34 [WejL5gpAID0AAFFXoCcAAACU] mw1191 wikidatawiki 1.31.0-wmf.4 error INFO: [WejL5gpAID0AAFFXoCcAAACU] /w/api.php ErrorException from line 122 of /srv/mediawiki/php-1.31.0-wmf.4/extensions/Wikidata/vendor/wikibase/data-model-serialization/src/Deserializers/SnakDeserializer.php: PHP Notice: Undefined index: value {"exception_id":"WejL5gpAID0AAFFXoCcAAACU","exception_url":"/w/api.php","caught_by":"mwe_handler"}
[Exception ErrorException] (/srv/mediawiki/php-1.31.0-wmf.4/extensions/Wikidata/vendor/wikibase/data-model-serialization/src/Deserializers/SnakDeserializer.php:122) PHP Notice: Undefined index: value
#0 /srv/mediawiki/php-1.31.0-wmf.4/extensions/Wikidata/vendor/wikibase/data-model-serialization/src/Deserializers/SnakDeserializer.php(122): MWExceptionHandler::handleError(integer, string, string, integer, array, array)
#1 /srv/mediawiki/php-1.31.0-wmf.4/extensions/Wikidata/vendor/wikibase/data-model-serialization/src/Deserializers/SnakDeserializer.php(114): Wikibase\DataModel\Deserializers\SnakDeserializer->deserializeDataValue(array)
#2 /srv/mediawiki/php-1.31.0-wmf.4/extensions/Wikidata/vendor/wikibase/data-model-serialization/src/Deserializers/SnakDeserializer.php(96): Wikibase\DataModel\Deserializers\SnakDeserializer->newValueSnak(array)
#3 /srv/mediawiki/php-1.31.0-wmf.4/extensions/Wikidata/vendor/wikibase/data-model-serialization/src/Deserializers/SnakDeserializer.php(78): Wikibase\DataModel\Deserializers\SnakDeserializer->getDeserialized(array)
#4 /srv/mediawiki/php-1.31.0-wmf.4/extensions/Wikidata/vendor/wikibase/data-model-serialization/src/Deserializers/SnakListDeserializer.php(60): Wikibase\DataModel\Deserializers\SnakDeserializer->deserialize(array)
#5 /srv/mediawiki/php-1.31.0-wmf.4/extensions/Wikidata/vendor/wikibase/data-model-serialization/src/Deserializers/SnakListDeserializer.php(41): Wikibase\DataModel\Deserializers\SnakListDeserializer->getDeserialized(array)
#6 /srv/mediawiki/php-1.31.0-wmf.4/extensions/Wikidata/extensions/Wikibase/repo/includes/Api/SetReference.php(120): Wikibase\DataModel\Deserializers\SnakListDeserializer->deserialize(array)
#7 /srv/mediawiki/php-1.31.0-wmf.4/includes/api/ApiMain.php(1584): Wikibase\Repo\Api\SetReference->execute()
#8 /srv/mediawiki/php-1.31.0-wmf.4/includes/api/ApiMain.php(546): ApiMain->executeAction()
#9 /srv/mediawiki/php-1.31.0-wmf.4/includes/api/ApiMain.php(517): ApiMain->executeActionWithErrorHandling()
#10 /srv/mediawiki/php-1.31.0-wmf.4/api.php(94): ApiMain->execute()
#11 /srv/mediawiki/w/api.php(3): include(string)
#12 {main}
```
#patch-for-review:
[] https://github.com/wmde/WikibaseDataModelSerialization/pull/241