Black color comes instead a transparent background for textview in android

The following two pics are a TextView with the following properties in it:

<TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:background="@drawable/button_style"
            android:text="select password"
            android:textColor="@color/dif_black"/>

and the button_style.xml is

<?xml version="1.0" encoding="UTF-8"?>
<layer-list >
<item>
    <shape>
        <stroke
            android:width="2dp"
            android:color="#88C425" />
        <corners android:radius="20dp" />
    </shape>
</item>

First pic taken from Canvas 2, Second pic taken from Samsung Galaxy Fame . Here is the problem I don't want the black color to be filled inside the textview border(Stroke). You would have noticed in the first pic that the background of the textview is transparent. I want this same in all android devices to be a transparent background.