https://es.wikipedia.org/wiki/Especial:Categor%C3%ADasRequeridas (Special:WantedCategories) returns a fatal exception with a text similar to:
[V@qQggpAAEIAADi6SMoAAABC] 2016-09-27 15:30:10: Excepción grave de tipo "Wikimedia\Assert\ParameterAssertionException"
Looks like the error only happens in the Spanish language Wikipedia.