Serializer for class is not found. Mark the class as @Serializable or provide the serializer explicitly

Question

Released APK crashes with next error:

Serializer for class 'User' is not found.
Mark the class as @Serializable or provide the serializer explicitly.

@Keep
@Serializable
data class User(
    val id: Long = 0,
    val name: String? = null,
    val token: String? = null
)

@ExperimentalSerializationApi
object UserSerializer : Serializer<User> {
    override val defaultValue: User
        get() = User()

    override suspend fun readFrom(input: InputStream): User {
        return ProtoBuf.decodeFromByteArray(input.readBytes())
    }

    override suspend fun writeTo(t: User, output: OutputStream) {
        output.write(ProtoBuf.encodeToByteArray(t))
    }
}

I added official proguard rules from

https://github.com/Kotlin/kotlinx.serialization#android

(with my package of course)

Even added @Keep to User class but still it crashes

What is the problem here?

UPDATE

Found the issue. I actually had one companion object inside User and had to add @Keep for this one as well to fix the issue:

@Keep
@Serializable
data class User(
    val id: Long = 0,
    val name: String? = null,
    val token: String? = null,
    ...
) {
    @Keep
    companion object {
        const val ...
        ...
    }
}