Google Cloud text-to-speech REST fetch error

Question

I'm trying to call the Google Cloud text-to-speech API from javascript based browser app. I'm getting this error response for my REST call:

error: Object { code: 400, message: "Empty language code.", status: "INVALID_ARGUMENT" }

the language code is correct according to the docs... i could be doing something else wrong if anyone can point me in the right direction? Here is my code:

const config = {
      method: 'POST',
      async: true,
      crossDomain: true,
      url: "https://texttospeech.googleapis.com/v1beta1/text:synthesize?fields=audioContent&key="+API_KEY,
      headers: {
          "x-goog-api-key": API_KEY,
          "Content-Type": "application/json; charset=utf-8",
          "cache-control": "no-cache"
      },
      processData: false,
      data: {
      'input':{
         'text':'This is a text to speak'
      },
      'voice':{
         'languageCode':'en-US',
         'name':'en-US-Standard-B'
      },
      'audioConfig':{
         'audioEncoding':'MP3'
       }
     }
}
let response = await window.fetch(config.url, config)
let data = await response.json()    
console.log("got buffer", data)