jalview desktop fails to launch with exception when using proxy

Standard out/error ike the following is observed when Jalview tries to start up:

Java version: 1.6.0_29
x86_64 Mac OS X 10.6.8

Using proxyServer: xx.yy.zz proxyPort: null
Exception in thread "main" java.lang.NullPointerException
        at java.util.Hashtable.put(Hashtable.java:394)
        at java.util.Properties.setProperty(Properties.java:143)
        at java.lang.System.setProperty(System.java:746)
        at jalview.bin.Cache.loadProperties(Cache.java:278)
        at jalview.bin.Jalview.main(Jalview.java:132)