KSerializer.serialize() of nullable type not invoked for null values

[koral--]

In the project's code there is a nullable value.
Magic not null values need to be deserialized to null.
And nulls need to be serialized to non-null magic values.

While I'm able to deserialize to nulls the serialization does not work. serialize() method is not fired and null is passed to serialized form.

To Reproduce
Serializer code:

internal object LocalTimeSerializer : KSerializer<LocalTime?> {

    override val descriptor = PrimitiveDescriptor("LocalTime?", PrimitiveKind.INT)

    override fun deserialize(decoder: Decoder): LocalTime? = when (val seconds = decoder.decodeInt()) {
       -1 -> null
        else -> LocalTime.ofSecondOfDay(seconds.toLong())
    }

    override fun serialize(encoder: Encoder, value: LocalTime?) {
        when (value) {
            null -> encoder.encodeInt(-1) //this line is never reached despite that nulls exist in serialized lists
            else -> encoder.encodeInt(value.toSecondOfDay())
        }
    }
}

Usage inside serializable class:

val times: List<@Serializable(with = LocalTimeSerializer::class) LocalTime?>,

Expected behavior
serialize() method is invoked for nulls.

Environment

• Kotlin version: 1.3.72
• Library version: 0.20.0
• Kotlin platforms: JVM
• Gradle version: 6.3+

[Dominaezzz]

Your serializer is probably being wrapped in another serializer that handles nulls.

[koral--]

I'm not explicitly wrapping it.
Serialization is used with Retrofit like that:

val converterFactory = Json(JsonConfiguration(ignoreUnknownKeys = true))
        .asConverterFactory("application/json".toMediaType())

Apart from enabling ignoreUnknownKeys there are no more customizations.

[vladrotea]

+1, I have the same issue.

[sandwwraith]

I think the issue happens because serializer is automatically wrapped with a standard NullableSerializer from the runtime library