How to fix ElasticSearch ‘Fielddata is disabled on text fields by default’ for keyword field

I'm getting the "Fielddata is disabled on text fields by default" on a keyword field. Below is the code.

{
 "aggs": {
   "agg_terms_user": {
     "terms": {
       "field": "user"
     }
   }
 }
}

The mapping for the user field is as below

user: { type: "keyword" }

Since the user field has type set as keyword I shouldn't get the error. However, the error is still thrown.

[illegal_argument_exception] Fielddata is disabled on text fields by default. Set fielddata=true on [user] in order to load fielddata in memory by uninverting the inverted index. Note that this can however use significant memory. Alternatively use a keyword field instead.

I don't know what to try now.

I found the reason behind the unexpected error. The ES wasn't getting re-indexed properly. Once I deleted the indexed first and then recreated it then it started working like a charm.