Accessing field in date_index_name processor

[ginger (jiangguoqiang)]

Hi.

I am using ingest node processor to overwrite index name, and I access field in date_index_name processor:

PUT _ingest/pipeline/monthlyindex
{
  "description": "monthly date-time index naming",
  "processors" : [
    {
      "date_index_name" : {
        "field" : "date1",
        "index_name_prefix" : "{{ _index }}-",
        "date_rounding" : "M"
      }
    }
  ]
}

When I put an event:
> PUT /myindex/type/1?pipeline=monthlyindex
> {
> "date1" : "2016-04-25T12:02:01.789Z"
> }

I get the following response:

{
  "error": {
    "root_cause": [
      {
        "type": "illegal_argument_exception",
        "reason": "Illegal pattern component: i"
      }
    ],
    "type": "illegal_argument_exception",
    "reason": "Illegal pattern component: i"
  },
  "status": 400
}

Access field in other processor works. Is there something wrong?
Any help would be appreciated!