gramophone has a problems with german umlauts ( Ä, Ö, Ü, ä, ö, ü, ß)

var gramophone = require('gramophone');
var fs = require('fs');

var result = gramophone.extract('Hallo Welt! Das ist ein Text über ganz viele Umlaute wie äöüÄÖÜß. Lörem Ipsüm Lörem Ipsüm.', {stopWords: JSON.parse(fs.readFileSync(__dirname + '/stopwords.json')) });
console.log(result);

result: [ 'rem ips m', 'l rem ips' ]

can you help me? maybe their is a problems with the utf8 encoding?