Missing Filter not working

I want to query the group_moel field that contains null value, but i am not getting the right result:
group_model is a field using default mapping setting. the result have records which group_model = "IS"

curl -X GET 'http://localhost:9200/manufactured_tires/manufactured_tire/_search?from=0&size=100&pretty' -d '{"filter":{"missing": {"field": "group_model", "null_value": null}}}'

Result:
{
"took" : 1,
"timed_out" : false,
"_shards" : {
"total" : 5,
"successful" : 5,
"failed" : 0
},
"hits" : {
"total" : 9,
"max_score" : 1.0,
"hits" : [ {
"_index" : "manufactured_tires-2013-03-04",
"_type" : "manufactured_tire",
"_id" : "6f0gOX7sRZu1gQjZUhlgXA",
"_score" : 1.0, "_source" : {"aspect_ratio":"40","car_designation":"E","load_range":"","load_rating_index":"93","make":"LEXUS","model":"IS F Base","primary_secondary":"1","radial_construction":"R","rim_diameter":"19","section_width":"225","speed_rating":"Y","tire_designation":"1","tire_size":"225/40R19","year":2011,"id":"6f0gOX7sRZu1gQjZUhlgXA","_type":"manufactured_tire","_score":0.0,"_index":"manufactured_tires-2013-03-04","_version":null,"sort":null,"highlight":null,"_explanation":null,"group_model":"IS"}
}, {
"_index" : "manufactured_tires-2013-03-04",
"_type" : "manufactured_tire",
"_id" : "ep9IXrlOR4Sqvek5c41HQg",
"_score" : 1.0, "_source" : {"aspect_ratio":"35","car_designation":"E","load_range":"","load_rating_index":"96","make":"LEXUS","model":"IS F Base","primary_secondary":"1","radial_construction":"R","rim_diameter":"19","section_width":"255","speed_rating":"Y","tire_designation":"2","tire_size":"255/35R19","year":2009,"id":"ep9IXrlOR4Sqvek5c41HQg","_type":"manufactured_tire","_score":0.0,"_index":"manufactured_tires-2013-03-04","_version":null,"sort":null,"highlight":null,"_explanation":null,"group_model":"IS"}

....