Firebase function test initialisation fails

Trying to test my Firebase cloud functions. I have followed the documentation and I get the following error when running the tests. I have tried generating different keys, I get to the service key window through Firebase so it is the correct project. The error occurs when I run the tests: npm test

Error: Options object {"_fieldsProto":{"foo":{"stringValue":"bar"}},"_ref":{"_firestore":{"_settings":{"credentials":{"private_key":"KEY","client_email":"project@appspot.gserviceaccount.com"},"projectId":"project","firebaseVersion":"8.13.0","libName":"gccl","libVersion":"3.8.6 fire/8.13.0"},"_settingsFrozen":false,"_serializer":{"allowUndefined":false},"_projectId":"project","registeredListenersCount":0,"_lastSuccessfulRequest":0,"_backoffSettings":{"initialDelayMs":100,"maxDelayMs":60000,"backoffFactor":1.3},"_preferTransactions":false,"_clientPool":{"concurrentOperationLimit":100,"maxIdleClients":1,"activeClients":{},"terminated":false,"terminateDeferred":{"promise":{}}}},"_path":{"segments":["document","path"],"projectId":"project","databaseId":"(default)"},"_converter":{}},"_serializer":{"allowUndefined":false},"_readTime":{"_seconds":1602169684,"_nanoseconds":285000000},"_createTime":{"_seconds":1602169684,"_nanoseconds":285000000},"_updateTime":{"_seconds":1602169684,"_nanoseconds":285000000}} has invalid key "_fieldsProto"
    at /home//Desktop/Work/test-app/app/functions/node_modules/firebase-functions-test/lib/main.js:99:19
    at Array.forEach (<anonymous>)
    at _checkOptionValidity (/home//Desktop/Work/test-app/app/functions/node_modules/firebase-functions-test/lib/main.js:97:26)
    at scheduledWrapped (/home//Desktop/Work/test-app/app/functions/node_modules/firebase-functions-test/lib/main.js:36:13)
    at Object.<anonymous> (/home//Desktop/Work/test-app/app/functions/index.test.js:98:1)
    at Module._compile (internal/modules/cjs/loader.js:1085:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:1114:10)
    at Module.load (internal/modules/cjs/loader.js:950:32)
    at Function.Module._load (internal/modules/cjs/loader.js:791:14)
    at Module.require (internal/modules/cjs/loader.js:974:19)
    at require (internal/modules/cjs/helpers.js:88:18)
    at Object.exports.requireOrImport (/home//Desktop/Work/test-app/app/functions/node_modules/mocha/lib/esm-utils.js:20:12)
    at Object.exports.loadFilesAsync (/home//Desktop/Work/test-app/app/functions/node_modules/mocha/lib/esm-utils.js:33:34)
    at Mocha.loadFilesAsync (/home//Desktop/Work/test-app/app/functions/node_modules/mocha/lib/mocha.js:427:19)
    at singleRun (/home//Desktop/Work/test-app/app/functions/node_modules/mocha/lib/cli/run-helpers.js:156:15)
    at exports.runMocha (/home//Desktop/Work/test-app/app/functions/node_modules/mocha/lib/cli/run-helpers.js:225:10)
    at Object.exports.handler (/home//Desktop/Work/test-app/app/functions/node_modules/mocha/lib/cli/run.js:366:11)
    at /home//Desktop/Work/test-app/app/functions/node_modules/yargs/lib/command.js:241:49


const test = require('firebase-functions-test')({
     databaseURL: "####",
     storageBucket: "####",
     projectId: "#####",   }, './key.json');
 
 
const myFunctions = require('./index.js');
 
const wrapped = test.wrap(myFunctions.testFunction);
const snap = test.firestore.makeDocumentSnapshot({foo: 'bar'}, 'document/path'); 
wrapped(snap); 
test.cleanup();

Followed this documentation: https://firebase.google.com/docs/functions/unit-testing

My package.json:

{
  "name": "functions",
  "description": "Cloud Functions for Firebase",
  "scripts": {
    "serve": "firebase emulators:start --only functions",
    "shell": "firebase functions:shell",
    "start": "npm run shell",
    "deploy": "firebase deploy --only functions",
    "logs": "firebase functions:log",
    "test": "mocha --reporter spec index.test.js"
  },
  "engines": {
    "node": "10"
  },
  "main": "index.js",
  "dependencies": {
    "firebase-admin": "^8.10.0",
    "firebase-functions": "^3.6.1",
    "reportdatasummary": "file:reportdatasummary"
  },
  "devDependencies": {
    "firebase-functions-test": "^0.2.0",
    "mocha": "^8.1.3",
    "reportdatasummary": "file:reportdatasummary"
  },
  "private": true
}

from Firebase function test initialisation fails