Verifying firebase custom token to get token ID fails when using jsonwebtoken

On the backend a custom token is generated via firebase's admin SDK thusly:

router.use('/get-token', (req, res) => {
    var uid = "big-secret";
    admin.auth().createCustomToken(uid)
      .then(function(customToken) {
        res.json({
          instanceID: customToken
        });
      })
      .catch(function(error) {
        console.log("Error creating custom token:", error);
    });
});

The client frontend app then picks up the customToken and with it makes a request back to the backend to verify:

const fbPrivateKey = serviceAccount.private_key;
const key = new NodeRSA(fbPrivateKey).exportKey('pkcs8-public-pem');
router.get('/verifyIdToken', cors(), (req, res) => {
  jwt.verify(req.headers.authorization.split('Bearer ')[1], key, { algorithms: ['RS256'] }, function(err, decoded) {
    console.log('err', err);
    console.log('decoded', decoded);
  });

This always errors with the message: JsonWebTokenError: invalid signature

Does this need signing? If anyone could explain this or has any pointers?

via Harry L