Using anchors.fill for a Canvas item produces a blank scene

import QtQuick 2.0

Rectangle {
   width: 300; height: 300

   Canvas {
       id: canvas
       //anchors.fill: parent      // produces a blank scene
       width: parent.width; height: parent.height      // ok

       onPaint: {
           var c = canvas.getContext('2d')
           c.strokeRect(125, 0, 50, 50)
       }
   }
}