the part.length is undefined and can't catch error

[friendsWhat]

var parse = require('co-busboy');
app.use(function* (next) {
  if (!this.request.is('multipart/*')) return yield next;
  var parts = parse(this, {
          checkFile: function (fieldname, filestream, filename) {
          if (path.extname(filename) !== '.dat') {
            return new Error('invalid filename extension');
          }
        }
   });
  var part;
try {
  while (part = yield parts) {
    if (part.length) {
      console.log('key: ' + part[0]);
      console.log('value: ' + part[1]);
    } else {
      part.pipe(fs.createWriteStream('some file.txt'))
    }
  }
catch (e) {
}
});

when upload a image file, the 'part' is FileStream instance.but it is no length method.
and then can't catch checkFile error. my request url's status always 'pending'.
i don't know why, pls help