Solr statistics

[Anil]

HI ,

I am using stats component to determine stats on two columns together.
stats query is failing with following exception -
*Invalid Number: num_useful  *


*Query response : *
{
  "responseHeader": {
    "zkConnected": true,
    "status": 400,
    "QTime": 19,
    "params": {
     * "q": "cid:HT201222",*
      "indent": [
        "true",
        "true"
      ],
      "stats": "true",
      "rows": [
        "0",
        "0"
      ],
      "wt": "json",
      "stats.field": "*{!func}termfreq('num_not_useful','num_useful')*"
    }
  },
  "error": {
    "metadata": [
      "error-class",
      "org.apache.solr.common.SolrException",
      "root-error-class",
      "org.apache.solr.common.SolrException"
    ],
    "msg": "Invalid Number: num_useful",
    "code": 400
  }
}

Could you please help me in resolving the issue? Thanks.

Regards,
Anil