TypeError: Expected Buffer

Using "bigchaindb-driver": "4.1.0", and run sample code:

const driver = require("bigchaindb-driver");

// BigchainDB server instance (e.g. https://example.com/api/v1/)
const API_PATH = "http://bigchain.local/api/v1";

// Create a new keypair.
const alice = new driver.Ed25519Keypair();

// Construct a transaction payload
const tx = driver.Transaction.makeCreateTransaction(
  // Define the asset to store, in this example it is the current temperature
  // (in Celsius) for the city of Berlin.
  { city: "Berlin, DE", temperature: 22, datetime: new Date().toString() },

  // Metadata contains information about the transaction itself
  // (can be `null` if not needed)
  { what: "My first BigchainDB transaction" },

  // A transaction needs an output
  [
    driver.Transaction.makeOutput(
      driver.Transaction.makeEd25519Condition(alice.publicKey)
    )
  ],
  alice.publicKey
);

// Sign the transaction with private keys
const txSigned = driver.Transaction.signTransaction(tx, alice.privateKey);

// Send the transaction off to BigchainDB
const conn = new driver.Connection(API_PATH);

conn
  .postTransactionCommit(txSigned)
  .then(retrievedTx =>
    console.log("Transaction", retrievedTx.id, "successfully posted.")
  );

It said:

Desktop/Capstone/bigchain-js is 📦 v1.0.0 via ⬢ v11.14.0 took 9s 
➜ node index.js
/Users/tuhuynh/Desktop/Capstone/bigchain-js/node_modules/base-x/index.js:29
    if (!Buffer.isBuffer(source)) throw new TypeError('Expected Buffer')
                                  ^

TypeError: Expected Buffer
    at Object.encode (/Users/tuhuynh/Desktop/Capstone/bigchain-js/node_modules/base-x/index.js:29:41)
    at new Ed25519Keypair (/Users/tuhuynh/Desktop/Capstone/bigchain-js/node_modules/bigchaindb-driver/dist/node/Ed25519Keypair.js:28:33)
    at Object.<anonymous> (/Users/tuhuynh/Desktop/Capstone/bigchain-js/index.js:7:15)
    at Module._compile (internal/modules/cjs/loader.js:816:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:827:10)
    at Module.load (internal/modules/cjs/loader.js:685:32)
    at Function.Module._load (internal/modules/cjs/loader.js:620:12)
    at Function.Module.runMain (internal/modules/cjs/loader.js:877:12)
    at internal/main/run_main_module.js:21:11

bigchaindb / js-bigchaindb-driver

TypeError: Expected Buffer #294