Transaction verification failed: Cannot update field 'appState' because permission for this field | Mina Protocol | Page 1

The full error is:

        Error trace: 

Cannot read properties of undefined (reading 'length')
    at checkPermission

Any idea what this means or how to fix?
The code is:

import {
  AccountUpdate,
  Mina,
  PrivateKey,
  PublicKey,
  fetchAccount,
  isReady,
  shutdown,
} from 'snarkyjs';
import { Add } from './Add.js';

await isReady;

const Local = Mina.LocalBlockchain({ proofsEnabled: true });
Mina.setActiveInstance(Local);

const { privateKey: deployerKey, publicKey: deployerAccount } =
  Local.testAccounts[0];
const { privateKey: senderKey, publicKey: senderAccount } =
  Local.testAccounts[1];

let zkAppPrivateKey = PrivateKey.random();
let appKey = zkAppPrivateKey.toPublicKey();
const zkApp = new Add(appKey);

const txn = await Mina.transaction(deployerKey, () => {
  AccountUpdate.fundNewAccount(deployerAccount);
  zkApp.deploy();
});
await txn.sign([deployerKey, zkAppPrivateKey]).send();

let num = zkApp.num.get();

console.log(num.toString());

// ----------------------------------------------------
await Add.compile();

const txn1 = await Mina.transaction(senderAccount, () => {
  zkApp.update();
});
await txn1.prove();
await txn1.sign([senderKey]).send();

num = zkApp.num.get();
console.log('state after txn1:', num.toString());
// ----------------------------------------------------
console.log('Shutting down');
await shutdown();

almost directly from the tutorial 1

#Transaction verification failed: Cannot update field 'appState' because permission for this field