<index>-_search script fields query response

{
  "error" : {
    "root_cause" : [
      {
        "type" : "script_exception",
        "reason" : "compile error",
        "script_stack" : [
          "doc[domain_name_length].value ...",
          "    ^---- HERE"
        ],
        "script" : "doc[domain_name_length].value * 2",
        "lang" : "painless"
      }
    ],
    "type" : "search_phase_execution_exception",
    "reason" : "all shards failed",
    "phase" : "query",
    "grouped" : true,
    "failed_shards" : [
      {
        "shard" : 0,
        "index" : "dns2",
        "node" : "l63dXeVTSeK1Xd18XxvrZQ",
        "reason" : {
          "type" : "script_exception",
          "reason" : "compile error",
          "caused_by" : {
            "type" : "illegal_argument_exception",
            "reason" : "Variable [domain_name_length] is not defined."
          },
          "script_stack" : [
            "doc[domain_name_length].value ...",
            "    ^---- HERE"
          ],
          "script" : "doc[domain_name_length].value * 2",
          "lang" : "painless"
        }
      }
    ],
    "caused_by" : {
      "type" : "script_exception",
      "reason" : "compile error",
      "caused_by" : {
        "type" : "illegal_argument_exception",
        "reason" : "Variable [domain_name_length] is not defined."
      },
      "script_stack" : [
        "doc[domain_name_length].value ...",
        "    ^---- HERE"
      ],
      "script" : "doc[domain_name_length].value * 2",
      "lang" : "painless"
    }
  },
  "status" : 500
}