How to add 2 values in elasticsearch script?

I am trying to create a rows_processed field by adding 2 fields src_s_rows and tgt_s_rows, but some how it is not working, it always gives me 0. Even when I give "script": "(doc['src_s_rows'].value)" instead of "script": "(doc['src_s_rows'].value+doc['tgt_s_rows'].value)" it still gives me 0.

What is it that I am missing, please help.

GET run_hist/task_hist/_search
{
   "fields": [
      "THROUGHPUT_ROWS_PER_SEC",
      "start_time",
      "end_time",
      "src_s_rows",
      "tgt_s_rows"
   ],
   "query": {
      "filtered": {
         "filter": {
            "bool": {
               "must": [
                  {
                     "term": {
                        "_id": "249885850"
                     }
                  }
               ]
            }
         }
      }
   },
   "filter": {
      "script": {
         "script": "(doc['end_time'].value-doc['start_time'].value)>minutes*1",
         "params": {
            "minutes": 60000
         }
      }
   },
   "script_fields": {
      "total_time_taken": {
         "script": "(doc['end_time'].value-doc['start_time'].value)/1000"
      },
      "rows_processed": {
         "script": "(doc['src_s_rows'].value+doc['tgt_s_rows'].value)"
      }
   },
   "size": 10000
}

Screenshot given below