ES SQL orderby groupby后的count做排序会报错

ES版本：6.6.1运行SQL：
POST _xpack/sql?format=txt
{
  "query":"select opIp,count(1) ipcount from systemlog   group by opIp order by ipcount"
}
 
报错信息：
{
  "error": {
    "root_cause": [
      {
        "type": "verification_exception",
        "reason": "Found 1 problem(s)\nline 1:70: Cannot order by non-grouped column [ipcount], expected [opIp]"
      }
    ],
    "type": "verification_exception",
    "reason": "Found 1 problem(s)\nline 1:70: Cannot order by non-grouped column [ipcount], expected [opIp]"
  },
  "status": 400
}