Elasticsearch completion field with jdbc river mapped data

I have made this mapping using jdbc river plugin for Elasticsearch:
localhost:9200/_river/product/_meta

{
"type": "jdbc",
"jdbc": {
"url": "jdbc:sqlserver://db.example.com;databaseName=training2",
"user": "root",
"password": "",
"sql": "SELECT uuid as uuid, caption as caption FROM caption_processed",
"index": "example",
"type": "product"
}
,
"type_mapping": {
"product" : {
"properties" : {
"caption" : {
"type" : "completion"
}
}
}
}}

And I made this query to test the suggest function:
localhost:9200//example/_suggest

{"product" : {
"text" : "",
"completion" : {
"field" : "content"
}}

}

This is the error that I get:

{
"_shards": {
"total": 5,
"successful": 0,
"failed": 5,
"failures": [
{
"index": "example",
"shard": 0,
"reason": "BroadcastShardOperationFailedException[[example][0] ]; nested: ElasticsearchException[failed to execute suggest]; nested: ClassCastException[org.elasticsearch.index.mapper.core.StringFieldMapper cannot be cast to org.elasticsearch.index.mapper.core.CompletionFieldMapper]; "
},
{
"index": "example",
"shard": 1,
"reason": "BroadcastShardOperationFailedException[[example][1] ]; nested: ElasticsearchException[failed to execute suggest]; nested: ClassCastException[org.elasticsearch.index.mapper.core.StringFieldMapper cannot be cast to org.elasticsearch.index.mapper.core.CompletionFieldMapper]; "
},

........................... }

Do you have any idea about what I'm doing wrongly?

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