Enum Issue with DynamoDB and .Net core 3.1

[vinaymalasiE]

I have enum property

public enum RequestUser{
None = 0,
Complaint = 1,
Appraisal = 2,
NewJob = 3,
Enquiry = 4,
}

Defining this Property in the Model class

 [JsonPropertyName("request_user")]
        [JsonConverter(typeof(StringNullableEnumConverter<RequestUser?>))] 
        public RequestUser? MessageType { get; set; } 

When I am trying to save this property in DynamoDB through .Net core its always throws Exception.

Type System.Nullable1[[Billing.Models.Enums.MessageTypeId, Billing.Models, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null]] is unsupported, it cannot be instantiated`

Does anyone know how can I save Enum into DynamoDB with .net core 3.1?

Thanks