Range query not formatted properly? (Elasticsearch)

Question

I am using the python elastic search client to query my elasticsearch index. My index is very small, because I am trying to deduce where my query is going wrong. Here is an example entry

_source: {
    Expiration: 20160115
}

and another

_source: {
    Expiration: 20160215
}

and another

_source: {
    Expiration: 20160315
}

Essentially, all the 15th's of each month for this year.

Here is my query/request

try:
    results = client.search(
        body = {
            'query' : {
                'range' : {
                    'Expiration' : {
                        'gte' : '2015',
                        'lte' : '2017',
                        'format' : 'yyyy'
                    }
                }
            }
        },
        index = 'test-index'
    )
except Exception as e:
    import pdb
    pdb.set_trace()

When I inevitably hit the break point in the exception, the exception is as follows.

RequestError(400, u'SearchPhaseExecutionException[Failed to execute phase [query], all shards failed; shardFailures {[xPPw7B5wTUu50pxUGxFGIQ][test-index][0]: RemoteTransportException[[guts_search_dev2][inet[/158.171.65.122:9301]][search/phase/query]]; nested: SearchParseException[[test-index][0]: from[-1],size[-1]: Parse Failure [Failed to parse source [{"query": {"range": {"Expiration": {"gte": "2015", "lte": "2017", "format": "yyyy"}}}}]]]; nested: QueryParsingException[[test-index] [range] query does not support [format]]; }{[cCrh939sR7yHdKgawRi6Sw][test-index][1]: SearchParseException[[test-index][1]: from[-1],size[-1]: Parse Failure [Failed to parse source [{"query": {"range": {"Expiration": {"gte": "2015", "lte": "2017", "format": "yyyy"}}}}]]]; nested: QueryParsingException[[test-index] [range] query does not support [format]]; }]', {u'status': 400, u'error': u'SearchPhaseExecutionException[Failed to execute phase [query], all shards failed; shardFailures {[xPPw7B5wTUu50pxUGxFGIQ][test-index][0]: RemoteTransportException[[guts_search_dev2][inet[/158.171.65.122:9301]][search/phase/query]]; nested: SearchParseException[[test-index][0]: from[-1],size[-1]: Parse Failure [Failed to parse source [{"query": {"range": {"Expiration": {"gte": "2015", "lte": "2017", "format": "yyyy"}}}}]]]; nested: QueryParsingException[[test-index] [range] query does not support [format]]; }{[cCrh939sR7yHdKgawRi6Sw][test-index][1]: SearchParseException[[test-index][1]: from[-1],size[-1]: Parse Failure [Failed to parse source [{"query": {"range": {"Expiration": {"gte": "2015", "lte": "2017", "format": "yyyy"}}}}]]]; nested: QueryParsingException[[test-index] [range] query does not support [format]]; }]'})

Something about "query does not support [format]" appears to be a common string.

I thought this might be an issue with the way I have mapped my index initially, so I deleted and remapped as follows.

{
    "test-index" : {
        "mappings" : {
            "properties" : {
                "Expiration" : {
                    "type" : "date",
                    "format" : "yyyy"
                }
            }
        }
    }
}

Still no luck. I am trying to do my best to follow this guide for my range queries. I don't know what I am doing wrong. Please help.

UPDATE : I removed "format" from my query, so that it now looks like this.

   'query' : {
            'range' : {
                'Expiration' : {
                    'gte' : '20160215',
                    'lte' : '20160415',
                }
            }
        }

And I got the desired entry. But why can I not use "format" as a param?