Could not cast or convert from System.String to Nest.CompletionField

[elastic_man]

Elasticsearch.Net and Nest 6.4.2

I am trying to implement completion suggestor in ASP.NET.

However, I am getting an exception error

Error converting value "200 1/2 E 111TH PL" to type 'Nest.CompletionField'. Path suggest.completion#address-suggest[0].options[0]._source.street_addr', line 29, position 50.

Here is my controller

var responsedata = _connectionToEs.EsClient().Search<address>(s => s
                                         .Index("address5")               
                                         .Suggest(ss => ss
                                            .Completion("address-suggest", cs => cs
                                                .Field(f => f
                                                    .street_addr)
                                                    .Prefix(phrase)
                                                    .Fuzzy(f => f
                                                        .Fuzziness(Fuzziness.Auto)
                                                    )
                                                    .Size(5)
                                            )
                                         ));

Here is my POCO

   public class address
    {
        public CompletionField street_addr { get; set; }
    }

My objective is to do autocomplete for addresses. Can anyone tell me what is wrong?