RxJava 2.x no longer accepts null values, as a simple workaround, an Observable<Null> can emit a Null, and it works with retrofit+Jackson when dealing with data from a web service.

Null.kt

import com.fasterxml.jackson.core.JsonParser
import com.fasterxml.jackson.databind.DeserializationContext
import com.fasterxml.jackson.databind.JsonDeserializer
import com.fasterxml.jackson.databind.annotation.JsonDeserialize

/**
 * null
 * Created by icris on 2017/3/26.
 */
@JsonDeserialize(using = NullDeserializer::class)
object Null

class NullDeserializer : JsonDeserializer<Null>() {
    override fun deserialize(p: JsonParser?, c: DeserializationContext?) = Null
    override fun getNullValue(c: DeserializationContext?) = Null
}