ClassCastException (от строки к числу) при попытке доступа к элементу в списке

У меня действительно странная проблема, которую я не могу понять.

Я просто пытаюсь получить доступ к элементу из списка с помощью простого геттера, но продолжаю получать исключение ClassCastException: String нельзя преобразовать в число. Однако в строке ошибки нет строк, а только Int, обращающийся к списку целых чисел. Исключение возникает в последней строке (я разделил их, чтобы отслеживать):

val cardKey1 = cards
val cardKey2 = cardKey1.keys
val cardKey3 = cardKey2.sorted()
val cardKey = cardKey3[1]

Насколько я проверял, cardKey3 — это список со значением [0,1] в условиях, которые я тестировал, а cardKey1 и cardKey2 — это MutableMap и MutableSet, если это поможет. При компиляции ошибок не появляется, только когда я открываю действие, вызывающее класс.

Полная ошибка выглядит следующим образом:

2021-02-28 22:29:59.629 12505-12505/rafaellobo.example.smartsave00 E/AndroidRuntime: FATAL EXCEPTION: main
    Process: rafaellobo.example.smartsave00, PID: 12505
    java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Number
        at rafaellobo.example.smartsave00.CardsAdapter.getView(CardsAdapter.kt:62)
        at android.widget.AbsListView.obtainView(AbsListView.java:2412)
        at android.widget.ListView.measureHeightOfChildren(ListView.java:1421)
        at android.widget.ListView.onMeasure(ListView.java:1327)
        at android.view.View.measure(View.java:24601)
        at androidx.constraintlayout.widget.ConstraintLayout$Measurer.measure(ConstraintLayout.java:792)
        at androidx.constraintlayout.solver.widgets.analyzer.BasicMeasure.measure(BasicMeasure.java:480)
        at androidx.constraintlayout.solver.widgets.analyzer.BasicMeasure.measureChildren(BasicMeasure.java:134)
        at androidx.constraintlayout.solver.widgets.analyzer.BasicMeasure.solverMeasure(BasicMeasure.java:277)
        at androidx.constraintlayout.solver.widgets.ConstraintWidgetContainer.measure(ConstraintWidgetContainer.java:119)
        at androidx.constraintlayout.widget.ConstraintLayout.resolveSystem(ConstraintLayout.java:1578)
        at androidx.constraintlayout.widget.ConstraintLayout.onMeasure(ConstraintLayout.java:1690)
        at android.view.View.measure(View.java:24601)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6831)
        at android.widget.FrameLayout.onMeasure(FrameLayout.java:194)
        at androidx.appcompat.widget.ContentFrameLayout.onMeasure(ContentFrameLayout.java:146)
        at android.view.View.measure(View.java:24601)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6831)
        at androidx.appcompat.widget.ActionBarOverlayLayout.onMeasure(ActionBarOverlayLayout.java:490)
        at android.view.View.measure(View.java:24601)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6831)
        at android.widget.FrameLayout.onMeasure(FrameLayout.java:194)
        at android.view.View.measure(View.java:24601)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6831)
        at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1552)
        at android.widget.LinearLayout.measureVertical(LinearLayout.java:842)
        at android.widget.LinearLayout.onMeasure(LinearLayout.java:721)
        at android.view.View.measure(View.java:24601)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6831)
        at android.widget.FrameLayout.onMeasure(FrameLayout.java:194)
        at com.android.internal.policy.DecorView.onMeasure(DecorView.java:776)
        at android.view.View.measure(View.java:24601)
        at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:3091)
        at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1912)
        at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:2207)
        at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1800)
        at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:7751)
        at android.view.Choreographer$CallbackRecord.run(Choreographer.java:967)
        at android.view.Choreographer.doCallbacks(Choreographer.java:791)
        at android.view.Choreographer.doFrame(Choreographer.java:726)
        at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:952)
        at android.os.Handler.handleCallback(Handler.java:883)
        at android.os.Handler.dispatchMessage(Handler.java:100)
        at android.os.Looper.loop(Looper.java:241)
        at android.app.ActivityThread.main(ActivityThread.java:7582)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:941)

Полный код класса выглядит следующим образом, если это поможет:

@RequiresApi(Build.VERSION_CODES.O)
class CardsAdapter(
    private val cardsContext: Context,
    private val resource: Int,
    private val cards: MutableMap<Int, Card>
) : ArrayAdapter<Transaction>(cardsContext, resource) {

    private val TAG = "CardsAdapter"
    private val inflater = LayoutInflater.from(context)

    override fun getCount(): Int {
        return cards.size
    }

    override fun getView(position: Int, convertView: View?, parent: ViewGroup): View {
        Log.d(TAG, "getView called")

        val view: View
        val viewHolder: ViewHolderCards

        if (convertView == null) {
            view = inflater.inflate(resource, parent, false)
            viewHolder = ViewHolderCards(view)
            view.tag = viewHolder
        } else {
            view = convertView
            viewHolder = view.tag as ViewHolderCards
        }

        val cardKey1 = cards
        val cardKey2 = cardKey1.keys
        val cardKey3 = cardKey2.sorted()
        val cardKey = cardKey3[1]
        val currentCard = cards[cardKey]

        viewHolder.txtCartao.text = currentCard?.cardNumber
        viewHolder.txtTipoInvestimento.text = currentCard?.getInvestmentType()

        viewHolder.txtEditar.setOnClickListener { _ ->
            val intent = Intent(cardsContext, EditCardActivity::class.java)
            intent.putExtra("cardId", cardKey)
            startActivity(cardsContext, intent, null)
        }

        viewHolder.txtExcluir.setOnClickListener { _ ->
            val builder = AlertDialog.Builder(context)
            builder.setTitle(context.getString(R.string.atencao))
            builder.setMessage(context.getString(R.string.confirmar_excluir))
            builder.setPositiveButton(context.getString(R.string.excluir)) { _, _ ->
                cards.remove(cardKey)
                notifyDataSetChanged()
                toast(context, context.getString(R.string.toast_cartao_excluido))
            }
            builder.setNegativeButton(context.getString(R.string.cancelar)) { _, _ -> }
            builder.show()
        }

        return view
    }
}