Error "null cannot be cast to non-null type kotlin.Any" decoding invalid json

[mamcx]

I trying to decode some JSON with com.beust:klaxon:5.5 and bail out If I get something invalid, however in Klaxon().parse I don't get a null but a inner exception:

 Caused by: java.lang.NullPointerException: null cannot be cast to non-null type kotlin.Any
    at com.beust.klaxon.JsonObjectConverter.calculatePolymorphicClass(JsonObjectConverter.kt:233)
    at com.beust.klaxon.JsonObjectConverter.initIntoUserClass(JsonObjectConverter.kt:55)
    at com.beust.klaxon.JsonObjectConverter.fromJson(JsonObjectConverter.kt:30)
    at com.beust.klaxon.DefaultConverter.fromJsonObject(DefaultConverter.kt:223)
    at com.beust.klaxon.DefaultConverter.fromJson(DefaultConverter.kt:40)
    at com.beust.klaxon.Klaxon.fromJsonObject(Klaxon.kt:296)

The code for trigger this:

data class Coordinates(val lat: Double, val long: Double)
data class JsonError(val kind: String, val message: String)

@TypeFor(field = "type", adapter = QueryTypeAdapter::class)
open class Query(val type: QueryType)
data class StartTravel(val from:Coordinates, val to:Coordinates): Query(QueryType.StartTravel)
data class Fail(val error:JsonError): Query(QueryType.Fail)

class QueryTypeAdapter: TypeAdapter<Query> {
    override fun classFor(type: Any): KClass<out Query> = when(type as QueryType) {
        QueryType.StartTravel-> StartTravel::class
        QueryType.Fail -> Fail::class
    }
}

    private fun decode(json:String):Query {
        val obj = Klaxon().parse<Query>(json) <--- HERE
        obj ?: return Fail(
            JsonError(
                kind = "Json",
                message = "Invalid JSON:$json"
            ))

        return obj
    }

and the invalid json:

{"Fail":{"kind":"JSON","message":"unknown variant `from`, expected one of `StartTravel`, `SearchingDriver`, `Fail` at line 1 column 7"}}