Spark doesn't work after upgrade to Java 7u51

I've been using spark happily with java 7 for some time now, but after the update to 51, spark goes 'black' -- that is, when its windows start to render, it only gets as far as rendering the bounding region, and then stops. This leaves a black rectangle the size of the spark window, which is unmoveable unless I force-quit the application.

My platform is Windows 7/64, with Java 7u51.

Thanks!