DynamoDB Table query items using global secondary index

I am trying to query a dynamo table with latitude and longitude for various locations. I want to get the values between certain coordinates as a user pans on the map.

The primary key for the table is city and the sort key is id. I created a global secondary index with lat as the partition key and lon as the sort key (to query for locations between two points in latitude and longitude).

I am trying to use this query:

let doc = require('dynamodb-doc');
let dynamo = new doc.DynamoDB();

...

        var params = {
            TableName : "locations-dev",
            IndexName: "lat-lon-index",
            KeyConditionExpression: "lon between :lon2 and :lon1 AND lat between :lat1 and :lat2",
            ExpressionAttributeValues: {
                ":lat1": JSON.stringify(event.bodyJSON.east),
                ":lat2": JSON.stringify(event.bodyJSON.west),
                ":lon1": JSON.stringify(event.bodyJSON.north),
                ":lon2": JSON.stringify(event.bodyJSON.south)
            }
        };

        dynamo.query(params, function (err, data) {
            if (err) {
                console.error('Error with ', err);
                context.fail(err);
            } else {
                context.succeed(data);
            }
        });

But I am getting this error:

{
   "errorMessage": "Query key condition not supported",
   "errorType": "ValidationException",
   "stackTrace": [
    ...
  ]
 }

Here is an example item in Dynamo:

{
  "id": "18",
  "lat": "39.923070",
  "lon": "-86.036178",
  "name": "Home Depot",
  "phone": "(317)915-8534",
  "website": "https://corporate.homedepot.com/newsroom/battery-recycling-one-million-pounds"
}

via Rose