Cloudant query error: missing search index when trying to sort

I've followed the Cloudant tutorial from IBM on how to run a query to retrieve a selection of documents, and sort these based on a field's value. The instructions start about 2/3rd way down the page at "Running a query with two fields".

I'm getting an error which I can't find a solution for.

Unknown Error: mango_idx :: {no_usable_index,missing_sort_index}

The query is:

{
"selector": {
  "lastname": "Brown",
  "location": "New York City, NY"
},
"fields": [
  "firstname",
  "lastname",
  "location"
],
"sort": [
  {
    "lastname": "asc"
  },
  {
    "firstname": "asc"
  }
]
}

I have a query index added to the design documents, as given:

{
"index": {
  "fields": [
    "lastname",  
    "location",
    "age"
  ]
},
"name": "query-index",
"type": "json"
}

The documents themselves are variants of:

{
"firstname": "John",
"lastname": "Brown",
"age": 21,
"location": "New York City, NY",
"_id": "doc2"
}

Am I doing something wrong? Did something change in the Cloudant syntax that the tutorial wasn't updated on?

I'm aware of "Unknown Error: mango_idx :: {no_usable_index,missing_sort_index}"} of which the suggested answers don't help me.

It does look like this is an issue with that tutorial. There is no index to support sorting by lastname and firstname. You can create another index like this:

{
  "index": {
    "fields": [
      "lastname",
      "firstname"
    ]
  },
  "type": "json"
}

Alternatively you could sort only by lastname (take out firstname). This will be supported by the original index. Query would look like this:

{
  "selector": {
    "lastname": "Brown",
    "location": "New York City, NY"
  },
  "fields": [
    "firstname",
    "lastname",
    "location"
  ],
  "sort": [
    {
      "lastname": "asc"
    }
  ]
}