TypeError: TypeError: path must be a string or Buffer when calling

I'm having trouble while calling Polly.

`Polly.synthesizeSpeech(params, (err, data) => {
if (err) {
console.log(err.code)
} else if (data) {
if (data.AudioStream instanceof Buffer) {
Fs.writeFile(argv.mp3, data.AudioStream, (err)=> {
if (err) {
return console.log(err)
}
console.log("The file was saved!")
var output = child_process.execSync('lame --decode ' + argv.mp3 + ' ' + '-b 8000' + ' ' + argv.wav + '.wav');
})
}
}
})
`

/opt/aws-nodejs-sample/node_modules/aws-sdk/lib/request.js:31
throw err;
^

TypeError: path must be a string or Buffer
at TypeError (native)
at Object.fs.open (fs.js:631:11)
at Object.fs.writeFile (fs.js:1303:6)
at Response. (/opt/aws-nodejs-sample/script.js:29:4)
at Request. (/opt/aws-nodejs-sample/node_modules/aws-sdk/lib/request.js:364:18)
at Request.callListeners (/opt/aws-nodejs-sample/node_modules/aws-sdk/lib/sequential_executor.js:105:20)
at Request.emit (/opt/aws-nodejs-sample/node_modules/aws-sdk/lib/sequential_executor.js:77:10)
at Request.emit (/opt/aws-nodejs-sample/node_modules/aws-sdk/lib/request.js:683:14)
at Request.transition (/opt/aws-nodejs-sample/node_modules/aws-sdk/lib/request.js:22:10)
at AcceptorStateMachine.runTo (/opt/aws-nodejs-sample/node_modules/aws-sdk/lib/state_machine.js:14:12)

I have no idea how to fix this.