Rounded Edges on a Button

[uksa]

I've been unable to set rounded edges on a button with st.corner_radius. So I've trued using the method suggested here:

http://stackoverflow.com/questions/9334618/rounded-button-in-android

In the folder resources/drawable/rounded_button.xml

<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android"
       android:shape="rectangle" android:padding="10dp">
  <corners
          android:bottomRightRadius="10dp"
          android:bottomLeftRadius="10dp"
          android:topLeftRadius="10dp"
          android:topRightRadius="10dp"/>
</shape>

I've added the background to RMQStyler

  def background=(background)
    @view.background = background
  end

So in my stylesheet I now have the reference:
st.background = RMQResource.find('drawable', 'rounded_button')

One build the app crashes, with no stack-trace. Is this an RubyMotion error with Android or have I done something really screwy!