CardView corners not rounded in an AlertDialog

I'm creating an AlertDialog and the custom layout for that dialog is a CardView (the card is the root of the xml layout):

<?xml version="1.0" encoding="utf-8"?>
<android.support.v7.widget.CardView
    xmlns:android="http://ift.tt/nIICcg"
    xmlns:app="http://ift.tt/GEGVYd"
    android:id="@+id/moreInfoCard"
    android:layout_gravity="center"
    app:cardCornerRadius="12dp"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content">

    //my layout content

</android.support.v7.widget.CardView>

When I show the dialog the card has its corners rounded, but the background of the corners it's not transparent.

How can I solve this issue? Thank you in advance.