nest - Elasticsearch - Term null? -

i'm having trouble match null term in nest. i'm trying details via query. works ok, 1 thing can't understand - reason can't term equals null value. doing wrong?

my code:

                result = _mainmanager.client.search<object>             (q => q              .type("mail")              .query(c =>                     c.term("sentmail_sender_id", userid) &&                     c.term("sentmail_embedaccountid", null) &&                     !c.term("sentmail_status", status.removed.tostring().tolower()) &&                     c.range(v => v.onfield("sentmail_upload_files_count").greater(0)))              .size(int.maxvalue)              .sort(s => s.onfield("sentmail_creation_date").descending())); 

it works ok, no null term found in result json:

 {  "size": 2147483647,  "sort": [  {    "sentmail_creation_date": {     "order": "desc"     }   } ], "query": { "bool": {    "must": [      {       "term": {         "sentmail_sender_id": {           "value": 7186         }       }     },     {       "range": {         "sentmail_upload_files_count": {           "gt": "0"         }       }     }   ],   "must_not": [     {        "term": {          "sentmail_status": {            "value": "removed"         }       }     }    ]   }  } } 

found it!

                result = _mainmanager.client.search<object>             (q => q              .type("mail")              .query(c =>                     c.term("sentmail_sender_id", userid) &&                     !c.term("sentmail_status", status.removed.tostring().tolower()) &&                     c.range(v => v.onfield("sentmail_upload_files_count").greater(0)))             .filter(f => f.missing("sentmail_embedaccountid"))             .size(int.maxvalue)             .sort(s => s.onfield("sentmail_creation_date").descending()));