Comando n dá certo

LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="br.com.tratosead.representantes.RepresentantesActivity">


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


    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="+"
        android:layout_alignParentRight="true"
        android:layout_alignParentBottom="true"
        android:layout_marginRight="16dp"
        android:layout_marginBottom="16dp"/>
        />


</LinearLayout>

não está dando certo. O botão n aparece