Aggregations returning null

here is my json file example that i indexed

{
    "DCMs": [
        {
            "00180088": {
                "vr": "DS",
                "Value": [
                    6
                ]
            }
           "12341234" : {..

and this is my aggregation that i am trying to use

{"size":0, "aggs":{"avg_00180088":{"avg":{"field":00180088"}}}}

but the return value that i get is:

{u'hits': {u'hits': [], u'total': {u'relation': u'eq', u'value': 1264}, u'max_score': None}, u'_shards': {u'successful': 1, u'failed': 0, u'skipped': 0, u'total': 1}, u'took': 1, u'aggregations': {u'avg_00180088': {u'value': None}}, u'timed_out': False}

can anyone help me please i am kinda newbie and realy need to figure something out

i am thinking about can i give nested field name like "00180088.Value" or
is it about the value in field is an array ?