MongoError: The dollar ($) prefixed field '$push' in '$push' is not valid for storage - node.js

Question

I am trying to upsert a dataset to a Mongo collection.
The intended document may or may not exist.
If it does exist, it will have at least one item in an embedded document (zips), and should append to that document rather than overwrite it.
If it does not exist, it should insert the new document to the collection.
When I run the below code, I am getting an error: MongoError: The dollar ($) prefixed field '$push' in '$push' is not valid for storage.
I put this together based on the docs: https://docs.mongodb.org/getting-started/node/update/#update-multiple-documents
Versions:
MongoDB (windows) = 3.2.0;
mongodb (npm package) = 2.1.4
var query = {
county: aCountyName,
state: aStateName
}
var params = {
'$set': {
county: 'Boone',
state: 'MO',
'$push': {
zips: {
'$each': [ '65203' ]
}
}
}
}
(could also be)
var params = {
'$set': {
county: 'Pierce',
state: 'WA',
'$push': {
zips: {
'$each': [ '98499', '98499' ]
}
}
}
}
db.collection(collectionName).updateMany(query, params, {'upsert': true},
function(err, results) {
callback();
}
);

Answer 1

I don't think $push is valid within a $set. Instead try adding it as another parameter, e.g.:
var params = {
'$set': {
county: 'Pierce',
state: 'WA'
},
'$push': {
zips: {
'$each': ['98499',
'98499']
}
}
}

Answer 2

The reason is because you didn't close the } so MongoDB think $push is a field's name and as mentioned in the documentation:
Field names cannot contain dots (i.e. .) or null characters, and they must not start with a dollar sign (i.e. $).
var query = {
county: aCountyName,
state: aStateName
};
var params = {};
params['$set'] = { county: 'Boone', state: 'MO' };
params['$push'] = { zips: { '$each': [ '65203' ] } };
Then:
db.collection(collectionName).updateMany(query, params, {'upsert': true},
function(err, results) {
callback();
}
);