Timelion series from cumulative sum of array length

I have documents, that look similar to below:

{
    dateTime: /* My time field */,
    message: {
        users: ['1', '2']
    },
    messageType: 'test'
}

I'd like to construct a timelion series chart that shows me a cumulative sum of the count of the array message.users. My first inkling was to create a script:

if(doc.containsKey('message.users')) {
    return doc['message.users'].length;
} else {
    return 0;
}

From what I could tell, doc.containsKey('message.users') always was false, which tells me that it may not have been indexed correctly. I've tried numerous Timelion, all to no avail:

.es(index=logstash-*,timefield='dateTime',q='messageType:UserList').label('Users Online')

I index my document through the c# NEST api like so:

elasticClient.Index(
    new
    {
        DateTime = DateTime.Now,
        Message = evt.EventArgs.Message,
    },
    idx => idx.Index($"logstash-{evt.MessageCode}"));