无效的参数（字符串）包含无效字符

Question

Printer.connect('192.168.0.20', port: 9100).then((printer) {
  printer.println('ကကကက');
  printer.cut();
  printer.disconnect();
  });

/////////////////错误//////////////////

invalid argument (string) contains invalid characters: "ကကကက"

如何解决此错误