TypeError: path must be a string with commander

I want to read a file with commander:

#!/usr/bin/env node

var fs = require('fs')
var program = require('commander')

program
  .version('0.0.1')
  .usage('<keywords>')
  .parse(process.argv)

if(!program.args.length) {
  program.help()
} else {
  var filename = program.args

  console.log('keys: ' + filename)

  fs.readFile(filename, 'utf8', function(err, data) {
    if (err) throw err
    console.log(data)
  })
}

$ ./m2k.js test.txt

However, I get the this error:

fs.js:430
  binding.open(pathModule._makeLong(path),
          ^
TypeError: path must be a string

What am I doing wrong?

(I have a file called test.txt in the same folder as m2k.js.)