Exception deserializing Json in Retrofit, "Polymorphic serializer was not found for missing class discriminator ('null')"

Working with Kotlin and Retrofit, I'm getting the following exception thrown on response to my http post, before onResponse()/onFailure() can get called:

kotlinx.serialization.json.internal.JsonDecodingException: Polymorphic serializer was not found for missing class discriminator ('null') JSON input: {"CustID":101,"DeviceID":81}

I'm just using a simple data class though, no polymorphism. What am I missing here?

My data object (I've added a lot of decoration trying to debug this):

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

My service:

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>
}

The call:

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")
             }
          })
    }
}

Reasonably new to Retrofit/Kotlin, I'm guessing I've missed something pretty obvious. Any ideas would be very appreciated.

Thanks