Button states with Background as AnimationDrawable in Android

I have made custom buttons in Android for a while. Things were simple, just made image resources for button states and made a selector for it. Everything went smooth and nice. Now I encountered a new situation. I have made a animation drawable and set it as a background for my button.

<animation-list xmlns:android="http://schemas.android.com/apk/res/android" android:oneshot="false">
  <item android:drawable="@drawable/frame1" android:duration="600" /> 
  <item android:drawable="@drawable/frame2" android:duration="300" /> 
  <item android:drawable="@drawable/frame3" android:duration="500" /> 
</animation-list> 

If I set the animation as button's Background it works fine. If I try to make a simple selector

<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android">
<item
        android:state_pressed="false"
        android:drawable="@drawable/animation" />

    <item
        android:state_pressed="true"
        android:drawable="@drawable/pressed" />
  </selector>      

where normal state of the button would have animation as background and the pressed state a static image, things don't work right.

On my main activity, on onWindowFocus I get the button background and start the animation

 @Override
  public void onWindowFocusChanged(boolean hasFocus) {
      super.onWindowFocusChanged(hasFocus);
          btn = (Button)findViewById(R.id.btnAnim);
          btnAnimation = (AnimationDrawable) btnAnim.getBackground();
          btnAnimation.start();
 }

Here appears to be the problem, because my animation won't be taken correctly from the selector and I get the following error:

03-14 15:21:16.146: ERROR/AndroidRuntime(440): FATAL EXCEPTION: main
03-14 15:21:16.146: ERROR/AndroidRuntime(440): java.lang.ClassCastException: android.graphics