问Retrofit中异常反序列化Json，“缺少类鉴别器(‘null’)没有找到多态序列化程序”

EN

@SerialName("NewCustVals")
@Serializable
data class NewCustVals (
    @SerialName("CustID")
    var CustID: Int? = 0,
    @SerialName("DeviceID")
    var DeviceID: Int? = 0
)

interface LTECustomerInterface {
    companion object {
        private var TAG :String = "LTECustomerInterface"
        var BASE_URL = "http://XXXXXXXXXXX:XXXX/"  //test server
        lateinit var callbackApi :ApiCallback

        @OptIn(ExperimentalSerializationApi::class)
        fun create(cb:ApiCallback): LTECustomerInterface {

            callbackApi = cb

            val retrofit = Retrofit.Builder()
                .baseUrl(BASE_URL)
                .addConverterFactory(Json{ignoreUnknownKeys = true; isLenient = true}.asConverterFactory( "application/json".toMediaType()))
             //   .client(client)
                .build()

            return retrofit.create(LTECustomerInterface::class.java)
        }
    }

    @POST("/newCustomer")
    suspend fun newCustomer(@Body data : Customer ) :Call<NewCustVals>
}

fun sendCustomer() {
    viewModelScope.launch(Dispatchers.IO) {
       val call = serverCustomerHttpInterface.newCustomer(_currentCust.value)

           call.enqueue(object : retrofit2.Callback<NewCustVals>{
             override fun onResponse(call: Call<NewCustVals>, response: Response<NewCustVals>) {
               Log.i(TAG,"NewCustomer onResponse called")
             }

             override fun onFailure(call: Call<NewCustVals>, t: Throwable) {
               Log.i("sendCustomer()","NewCustomer onFailure")
             }
          })
    }
}