The MLocale docs state: "As can be seen in the above examples, this function tries to return useful extra buckets for strings which sort out of the range of the bucket list of the language of the current locale." -- i.e. if a grouper is using MLocale, it either has to verify internally that it doesn't return anything outside of the group list, or seasidecache needs to verify this. I thought adding the check here would be extra insurance in case of a badly behaving grouper.