NPE when logging in

After logging in from the log in page (rather than clicking play) with an existing user I received this error:

[14:50:00 FATAL]: Fatal error launching game. Report this to http://mojang.atlassian.net please!
java.lang.NullPointerException
	at net.minecraft.launcher.GameLauncher.getMinecraftArguments(GameLauncher.java:315) ~[launcher.jar:?]
	at net.minecraft.launcher.GameLauncher.launchGame(GameLauncher.java:250) ~[launcher.jar:?]
	at net.minecraft.launcher.GameLauncher.onDownloadJobFinished(GameLauncher.java:521) [launcher.jar:?]
	at net.minecraft.launcher.updater.download.DownloadJob.popAndDownload(DownloadJob.java:116) [launcher.jar:?]
	at net.minecraft.launcher.updater.download.DownloadJob.access$000(DownloadJob.java:11) [launcher.jar:?]
	at net.minecraft.launcher.updater.download.DownloadJob$1.run(DownloadJob.java:86) [launcher.jar:?]
	at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source) [?:1.7.0_21]
	at java.util.concurrent.FutureTask$Sync.innerRun(Unknown Source) [?:1.7.0_21]
	at java.util.concurrent.FutureTask.run(Unknown Source) [?:1.7.0_21]
	at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source) [?:1.7.0_21]
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source) [?:1.7.0_21]
	at java.lang.Thread.run(Unknown Source) [?:1.7.0_21]

Its not really that much of a problem as I can still play by clicking the play button on the main page, but I thought I ought to report it anyway