NotSupported exception when dictionary key is union (DU) #177

{
  "key-values": {
    "Key1": {
      "string-value": "A",
      "int-value": 1
    },
    "Key2": {
      "string-value": "B",
      "int-value": 2
    },
    "Key3": {
      "string-value": "C",
      "int-value": 3
    }
  }
}

type SampleKey =
    | Key1
    | Key2
    | Key3
    
type SampleValue =
    { StringValue: string
      IntValue: int }
    
type SampleMap =
    { KeyValues: Map<SampleKey, SampleValue> }
    
type SampleKeyConverter() =
    inherit JsonConverter<SampleKey>()

    let read (reader: Utf8JsonReader) =
        let value = reader.GetString().ToLowerInvariant()
        match value with
        | "key1" -> Key1
        | "key2" -> Key2
        | "key3" -> Key3
        | _ -> failwith "Not supported"

    override _.Write(writer: Utf8JsonWriter, value: SampleKey, options: JsonSerializerOptions) =        
        value.ToString() |> writer.WriteStringValue

    override _.Read(reader: byref<Utf8JsonReader>, typeToConvert: System.Type, options: JsonSerializerOptions) =
        read reader

    override _.WriteAsPropertyName(writer: Utf8JsonWriter, value: SampleKey, options: JsonSerializerOptions) =
        value.ToString () |> writer.WritePropertyName

    override _.ReadAsPropertyName(reader: byref<Utf8JsonReader>, typeToConvert: System.Type, options: JsonSerializerOptions) =
        read reader

let jsonOptions =       
    let options =
        JsonFSharpOptions.FSharpLuLike()            
            .WithUnionTagCaseInsensitive()
            .WithMapFormat(MapFormat.Object)
            .ToJsonSerializerOptions()
    options.Converters.Insert(0, SampleKeyConverter())
    options.PropertyNamingPolicy <- JsonNamingPolicy.KebabCaseLower
    options

let keyValues =
    [ Key1, { StringValue = "A"; IntValue = 1 }
      Key2, { StringValue = "B"; IntValue = 2 }
      Key3, { StringValue = "C"; IntValue = 3 } ]
    |> Map.ofList
    
let testMap = { KeyValues = keyValues }

let jsonString = JsonSerializer.Serialize(testMap, jsonOptions)
printfn "%s" jsonString

let testMapBack =JsonSerializer.Deserialize<SampleMap>(jsonString, jsonOptions)
let test = testMap = testMapBack