Find and update a subdocument if specific key is null

I have a collection that will, at times, have a subdocument where a key will be null. Something like:

    _id: <some id>,
    taskTimes: [{
        startDateTime: <some ISO date time>,
        endDateTime: <some ISO data time>
        startDateTime: <some ISO date time>,
        endDateTime: null
    client: "my client"

I’m trying to figure out how to update the subdocument to add the endDateTime where it’s currently null.

I tried a solution with $ifNull, but minimongo complains about it. Maybe I have it entered wrong.

return Tasks.update({ _id: taskId }, [{
            $set: {
                'taskTimes.endDateTime': {
                    $ifNull: {
                        endDateTime: endDateTime,

I get a web browser console error:

Uncaught MinimongoError: Key $ifNull must not start with '$'
    at MinimongoError (common.js:1085:17)
    at assertIsValidFieldName (local_collection.js:1922:11)
    at Object.<anonymous> (local_collection.js:1913:7)
    at JSON.stringify (<anonymous>)
    at assertHasValidFieldNames (local_collection.js:1912:10)
    at LocalCollection.insert (local_collection.js:102:5)
    at Object.update (collection.js:239:28)
    at store.<computed> [as update] (livedata_connection.js:312:38)
    at livedata_connection.js:1340:19
    at Array.forEach (<anonymous>)

As always, any help is greatly appreciated.