jetpack compose - Kotlinx serialization not encode null values

MainActivity.kt 

package com.cfsuman.composenetwork

import android.os.Bundle
import androidx.activity.ComponentActivity
import androidx.activity.compose.setContent
import androidx.compose.foundation.layout.*
import androidx.compose.material.*
import androidx.compose.runtime.*
import androidx.compose.ui.Modifier
import androidx.compose.ui.unit.dp
import com.cfsuman.composenetwork.ui.theme.ComposeNetworkTheme
import kotlinx.serialization.ExperimentalSerializationApi
import kotlinx.serialization.Serializable
import kotlinx.serialization.encodeToString
import kotlinx.serialization.json.Json


class MainActivity : ComponentActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContent {
            ComposeNetworkTheme {
                Scaffold(
                    topBar = {
                        TopAppBar(
                            title = {
                                Text("Serialization Not Encode Null Values")
                            }
                        )
                    },
                    content = { MainContent()}
                )
            }
        }
    }
}


@OptIn(ExperimentalSerializationApi::class)
@Composable
fun MainContent() {
    val format = Json{
        prettyPrint = true
        // null values are not encoded into json
        explicitNulls = false
    }

    val user = User(firstName = "Suman", lastName = null, age = 20)
    val userString = format.encodeToString(user)

    Column(Modifier.fillMaxSize().padding(24.dp)) {
        Text(
            text = userString,
            style = MaterialTheme.typography.h5
        )
    }
}


@Serializable
data class User(
    val firstName: String,
    val lastName: String?,
    val age: Int
)
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