See this [sage-devel](https://groups.google.com/forum/#!topic/sage-devel/QgD6xb3oCEU) thread, and especially [this post](https://groups.google.com/d/msg/sage-devel/QgD6xb3oCEU/YIZALuSKBwAJ) which is my understanding of the problem at the moment.
See this sage-devel thread, and especially this post which is my understanding of the problem at the moment.