Error here TypeError [ERR_INVALID_ARG_TYPE]: The "path" argument must be of type string. Received type object

Hi I am trying to upload a base 64 file from NodeJs to firebase storage(google cloud storage) using the below given code. But I am getting an error stating

Error here TypeError [ERR_INVALID_ARG_TYPE]: The "path" argument must be of type string. Received type object at validateString (internal/validators.js:125:11)

the code is:

const key = require('../key.json');
const {Storage} = require('@google-cloud/storage');
//var gcs = require('@google-cloud/storage');
exports.uploadVideo = functions.https.onRequest((req, res) => {

    const gcs =new Storage({
        projectId: "<my-project-id>",
        keyFilename: key
      });

      const bucket = gcs.bucket("example.appspot.com");
      const gcsname = 'test.pdf';
      const file = bucket.file(gcsname);
      var pdfdata = "xNC9YUmVmU3RtIDE1NzQ+Pg0Kc3RhcnR4cmVmDQoyMTY5DQolJUVPRg==";
      var buff = Buffer.from(pdfdata, 'binary').toString('utf-8');
      const stream = file.createWriteStream({
        metadata: {
          contentType: 'application/pdf'
        }
      });

      stream.on('error', (err) => {
        console.log("Error here",err);
      });
      stream.on('finish', () => {
        console.log(gcsname);
      });
      stream.end(new Buffer.from(buff, 'base64'));})

Can someone suggest me a way out? I can't understand the problem. Thanks