Creating task fails

2021-04-26T04:23:39.937466+00:00 app[web.1]: {"level":"error","message":"Error in creating task Value for argument \"data\" is not a valid Firestore document. Cannot use \"undefined\" as a Firestore value (found in field \"assignee\"). If you want to ignore undefined values, enable `ignoreUndefinedProperties`.","stack":"Error: Value for argument \"data\" is not a valid Firestore document. Cannot use \"undefined\" as a Firestore value (found in field \"assignee\"). If you want to ignore undefined values, enable `ignoreUndefinedProperties`.\n    at validateUserInput (/app/node_modules/@google-cloud/firestore/build/src/serializer.js:271:19)\n    at Object.validateUserInput (/app/node_modules/@google-cloud/firestore/build/src/serializer.js:263:13)\n    at Object.validateDocumentData (/app/node_modules/@google-cloud/firestore/build/src/write-batch.js:577:18)\n    at CollectionReference.add (/app/node_modules/@google-cloud/firestore/build/src/reference.js:1957:23)\n    at Object.updateTask (/app/models/tasks.js:26:39)\n    at runMicrotasks (<anonymous>)\n    at processTicksAndRejections (internal/process/task_queues.js:93:5)\n    at async addNewTask (/app/controllers/tasks.js:16:18)"}
2021-04-26T04:23:39.937692+00:00 app[web.1]: {"message":"Error while creating new task: Error: Value for argument \"data\" is not a valid Firestore document. Cannot use \"undefined\" as a Firestore value (found in field \"assignee\"). If you want to ignore undefined values, enable `ignoreUndefinedProperties`.","level":"error"}
2021-04-26T04:23:39.939108+00:00 app[web.1]: {"message":"::ffff:10.38.10.19 - - [26/Apr/2021:04:23:39 +0000] \"POST /tasks HTTP/1.1\" 500 96 \"https://dashboard.realdevsquad.com/\" \"Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/90.0.4430.85 Safari/537.36\"\n","level":"info"}
2021-04-26T04:23:39.943104+00:00 heroku[router]: at=info method=POST path="/tasks" host=api.realdevsquad.com request_id=c3edf7c9-dccc-44a5-8867-2d93e78fa380 fwd="174.127.238.68" dyno=web.1 connect=0ms service=594ms status=500 bytes=656 protocol=https
2021-04-26T04:23:50.553861+00:00 app[web.1]: {"message":"::ffff:10.37.108.111 - - [26/Apr/2021:04:23:50 +0000] \"OPTIONS /tasks HTTP/1.1\" 200 0 \"https://dashboard.realdevsquad.com/\" \"Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/90.0.4430.85 Safari/537.36\"\n","level":"info"}
2021-04-26T04:23:50.558325+00:00 heroku[router]: at=info method=OPTIONS path="/tasks" host=api.realdevsquad.com request_id=8d0c8623-4afe-4554-badb-8d4edb0aeb81 fwd="174.127.238.68" dyno=web.1 connect=0ms service=3ms status=200 bytes=540 protocol=https
2021-04-26T04:23:51.296738+00:00 app[web.1]: {"level":"error","message":"Error in creating task Value for argument \"data\" is not a valid Firestore document. Cannot use \"undefined\" as a Firestore value (found in field \"assignee\"). If you want to ignore undefined values, enable `ignoreUndefinedProperties`.","stack":"Error: Value for argument \"data\" is not a valid Firestore document. Cannot use \"undefined\" as a Firestore value (found in field \"assignee\"). If you want to ignore undefined values, enable `ignoreUndefinedProperties`.\n    at validateUserInput (/app/node_modules/@google-cloud/firestore/build/src/serializer.js:271:19)\n    at Object.validateUserInput (/app/node_modules/@google-cloud/firestore/build/src/serializer.js:263:13)\n    at Object.validateDocumentData (/app/node_modules/@google-cloud/firestore/build/src/write-batch.js:577:18)\n    at CollectionReference.add (/app/node_modules/@google-cloud/firestore/build/src/reference.js:1957:23)\n    at Object.updateTask (/app/models/tasks.js:26:39)\n    at runMicrotasks (<anonymous>)\n    at processTicksAndRejections (internal/process/task_queues.js:93:5)\n    at async addNewTask (/app/controllers/tasks.js:16:18)"}
2021-04-26T04:23:51.297117+00:00 app[web.1]: {"message":"Error while creating new task: Error: Value for argument \"data\" is not a valid Firestore document. Cannot use \"undefined\" as a Firestore value (found in field \"assignee\"). If you want to ignore undefined values, enable `ignoreUndefinedProperties`.","level":"error"}

Start at models/tasks.js:26 and trace the task-creation path from controllers/tasks.js:16 for POST /tasks. Reproduce the failure and inspect how the assignee value reaches the Firestore document. Done means creating a task no longer returns the logged 500 error when assignee is absent.