Aula 3 Utilizando menus , vídeo 01 Floating button

Olá!

Por algum motivo o shadow do botão não está funcionando. Poderiam ver o que houve?

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:orientation="vertical"
    android:id="@+id/content_lista_alunos"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    app:layout_behavior="@string/appbar_scrolling_view_behavior"
    tools:context="com.rafaelamarraschi.agendaalunos.agendaalunos.ListaAlunosActivity"
    tools:showIn="@layout/activity_lista_alunos"
    android:clipToPadding="false">

<ListView
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:id="@+id/lista_alunos"
    />

    <Button
        android:layout_width="56dp"
        android:layout_height="56dp"
        android:text="+"
        android:layout_alignParentBottom="true"
        android:layout_alignParentRight="true"
        android:layout_marginBottom="16dp"
        android:layout_marginRight="16dp"
        android:background="@drawable/fundo"
        android:textColor="#fff"
        android:textSize="40sp"
        android:elevation="60dp"
        android:stateListAnimator="@null"

        />


</RelativeLayout>