ImageButton background with XML

I am trying to make a round Android ImageButton. To do so, I write the following code according to the link.

In my main.xml

<ImageButton
     android:layout_width="wrap_content"
     android:layout_height="wrap_content"
     android:id="@+id/imageButton"
     android:src="@drawable/round"
     android:background="@drawable/roundcorner"
     android:padding="50dp"  />

In my res/drawable/roundcorner.xml

<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://ift.tt/nIICcg">
    <solid android:color="#33DDFF" />
    <corners android:radius="100dp" />
</shape>

But the code doesn't work. The output is like below

However, when I change ImageButton to ImageView in main.xml

<ImageView
     android:layout_width="wrap_content"
     android:layout_height="wrap_content"
     android:id="@+id/imageButton"
     android:src="@drawable/round"
     android:background="@drawable/roundcorner"
     android:padding="50dp"  />

It works as below

I am confused since as I know ImageButton inherits from ImageView. Why does it work differently? Is there anyway to fix this? Thanks in advance!