When trying to perform sendSignedTransaction iteratively on wallets in an array, an undefined error is returned

Question

I'm trying to write a script that takes as input a JSON array where each element of the array has an address and a corresponding private key. I'd like to have a function that iterates through this array, checking the balance of each account and subsequently emptying it into an arbitrary external account.

However, when I run this function below, an error with no contents (undefined error) is returned. How can I figure out what's going wrong?

Here's my code, including sample input:

var Tx = require('ethereumjs-tx');

var wallets = [
  { _id: 5b898c5cda06f4c58c8aa59c,
    address: '0x107396c9f484666d0C43F227FEd9f3d258576F37',
    key:
     '0x4482be4e4e36d5521d70dafa57818eed38a4d0c69562bffebf24901c3199b271' },
  { _id: 5b89a45918f5dbce22e9115f,
    address: '0xB6125B6F5854547da1CF3B2CDe831cBEB7Ea31c0',
    key:
     '0x62305fc1c775f69603b1f378b5cd72815c102fb77d549d3364df7c9b53a0b444' } ]

var counter = 0;
var to = "0xa1cc63e76dc8dde54abf848810abd2680617b900";
var updatedBalance = 0;
var txHashes = [];

 function iterateAndEmptyWallets(counter, wallets, updatedBalance, txHashes, to) {

  if (counter >= wallets.length) {
    return [updatedBalance, txHashes]
  }

  var addr = wallets[counter].address;
  var key = wallets[counter].key;

  web3.eth.getBalance(addr).then( (result) => {
    result = parseInt(result, 10);
    var gasLimit = 25000;

    web3.eth.getGasPrice()
    .then((gasPrice) => {
      web3.eth.getTransactionCount(addr).then( (nonce) => {
        var nonce = nonce.toString();
        var valAfterGas = result - (gasPrice * gasLimit)

        key = key.slice(2);

        var rawTransaction = {
          "from": addr,
          "nonce": web3.utils.toHex(nonce),
          "gasPrice": web3.utils.toHex(gasPrice),
          "gasLimit": web3.utils.toHex(gasLimit),
          "to": to,
          "value": valAfterGas,
        };

        var privKey = Buffer.from(key, 'hex');

        var tx = new Tx(rawTransaction);

        tx.sign(privKey);
        var serializedTx = tx.serialize();
        updatedBalance += valAfterGas;
        var transaction = web3.eth.sendSignedTransaction('0x' + serializedTx.toString('hex'));

        transaction.on('receipt', receipt => {
          txHashes.push(receipt.transactionHash);
          counter++;
          iterateAndEmptyWallets(counter, wallets, updatedBalance, txHashes, to);
        });
        transaction.on('error', error => {
          return error
        });
      });
    });
  });
}