Android shrink and grow sequential animation

I want to make a sequential animation in which two buttons shrink until they disappear and then they grow again to their original size. When I run it the buttons simply disappear without any animations

<?xml version="1.0" encoding="utf-8"?>
<set xmlns:android="http://schemas.android.com/apk/res/android" 
    android:interpolator="@android:anim/accelerate_decelerate_interpolator"
    android:fillAfter="true">

    <scale
        android:fromXScale="1"
        android:toXScale="0"
        android:fromYScale="1"
        android:toYScale="0"
        android:duration="400"
        android:pivotX="50%"
        android:pivotY="50%"
        /> 

    <scale
        android:startOffset="700"
        android:fromXScale="0"
        android:toXScale="1"
        android:fromYScale="0"
        android:toYScale="1"
        android:duration="400"
        android:pivotX="50%"
        android:pivotY="50%"
     /> 

</set>  

Animation sgAnimation = AnimationUtils.loadAnimation(getApplicationContext(), R.anim.shrink_grow);
btPrimary.startAnimation(sgAnimation);
btSecondary.startAnimation(sgAnimation);