Cannot cast RuntimeAnimatorController to AnimatorOverrideController

I want to use AnimatorOverrideController to change the animation at run time through script. I tried this which gives and InvalidCastException:

 RuntimeAnimatorController controller = animator.runtimeAnimatorController;
 AnimatorOverrideController overrideController = (AnimatorOverrideController)controller;

And this makes controller to null

 RuntimeAnimatorController controller = animator.runtimeAnimatorController;
 AnimatorOverrideController overrideController = controller as AnimatorOverrideController;

That's what i found in all the code samples everywhere. Not sure what is going wrong here. I am using Unity 5.5