App crashing when trying to save a file using ObjectOutputStream

MainActivity looks like this:

    turma.alunos.add(Aluno("Weslley", "123"))
    turma.alunos.add(Aluno("Thiago", "1235"))
    turma.alunos.add(Aluno("Thayane", "1234"))
    turma.alunos.add(Aluno("Kelvin", "1253"))
    turma.alunos.add(Aluno("Carlos", "12253"))

    var arq = ArquivoUtils(turma, this.applicationContext)

    recycler_view.layoutManager = LinearLayoutManager(this)
    recycler_view.adapter = AlunoAdapter(this,turma.alunos)

Class responsible for handling the file

class ArquivoUtils internal constructor( var turminha: Turma, var context : Context) {

internal val fos = this.context.openFileOutput("turma.dat", Context.MODE_PRIVATE)
internal val oos = ObjectOutputStream(fos)

init {
    //this.criarArquivo()
    this.salvarArquivo()
    this.closeFile()
}

@Throws(IOException::class)
private fun salvarArquivo() {
    oos.writeObject(turminha)
}

@Throws(IOException::class)
fun closeFile() {
    oos.close()
}

Logcat:

  --------- beginning of crash
09-22 01:09:07.626 2601-2601/? E/AndroidRuntime: FATAL EXCEPTION: main
                                             Process: com.example.thial.estudandokotlin, PID: 2601
                                             java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.thial.estudandokotlin/com.example.thial.estudandokotlin.MainActivity}: java.io.NotSerializableException: com.example.thial.estudandokotlin.Aluno
                                                 at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2646)
                                                 at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707)
                                                 at android.app.ActivityThread.-wrap12(ActivityThread.java)
                                                 at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1460)
                                                 at android.os.Handler.dispatchMessage(Handler.java:102)
                                                 at android.os.Looper.loop(Looper.java:154)
                                                 at android.app.ActivityThread.main(ActivityThread.java:6077)
                                                 at java.lang.reflect.Method.invoke(Native Method)
                                                 at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865)
                                                 at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)

The error is exactly in the save method, when I do not run it works normally.