ethereum - web3 を使用した Object.exports.toBuffer の無効な型

Question

Truffle 経由で生のトランザクションをガナッシュに送信しようとしています。エラー メッセージ「Object.exports.toBuffer の型が無効です」。

コードは次のとおりです。

const Tx = require('ethereumjs-tx').Transaction;
const privateKey = Buffer.from('5b58ae3fe5bf48d05f0a94c87a28fb480ec2bc200a4423ab42ea9557f3617743', 'hex');

const payment_usercashTx = {
    nonce: myAsyncNonceFunction(),
    gasPrice: web3.utils.toHex(25000),
    gasLimit: web3.utils.toHex(10e9),
    to: '0x107180312267DB452930731EA475187BdD4948DB',
    from: web3.eth.defaultAccount,
    value: '0x00',
    data: encode_usercash
}

let usercashTx = new Tx(payment_usercashTx);
usercashTx.sign(privateKey);

let usercash_serializedTx = usercashTx.serialize();

web3.eth.sendSignedTransaction('0x' + usercash_serializedTx.toString('hex'))
.on('receipt', console.log);

新しいメッセージ：

Error: invalid type
    at Object.exports.toBuffer (/Users/yuha/Desktop/blockchainProject/Kidari/node_modules/ethereumjs-util/dist/bytes.js:97:19)
    at Transaction.setter [as nonce] (/Users/yuha/Desktop/blockchainProject/Kidari/node_modules/ethereumjs-util/dist/object.js:43:25)
    at /Users/yuha/Desktop/blockchainProject/Kidari/node_modules/ethereumjs-util/dist/object.js:96:38
    at Array.forEach (<anonymous>)
    at Object.exports.defineProperties (/Users/yuha/Desktop/blockchainProject/Kidari/node_modules/ethereumjs-util/dist/object.js:94:20)
    at new Transaction (/Users/yuha/Desktop/blockchainProject/Kidari/node_modules/ethereumjs-tx/dist/transaction.js:127:27)
    at Object.<anonymous> (/Users/yuha/Desktop/blockchainProject/Kidari/routes/payment.js:72:18)