Can't set alpha of ImageView in XML
===

I'm compiling using API 16, but with the minimum supported API set to 8. I'm trying to set the `alpha` of an `ImageView` in XML, but every time I use 

    <ImageView 
        android:id="@+id/image"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:alpha="0"
        .
        .
        . />

AAPT complains that alpha isn't a valid attribute, even if the documentation in the android site states that `ImageView's` supposed to inherit it from View. 

Is this a bug, or is this really how it's supposed to work. I'd set the Visibility, but that won't work well with my animations to show and hide the image.