Android refusing to build .apk file

So I press build, it runs for bit, then asks me to select the root folder for my SDK. I do so, but then it just exits without any popups or anything. After trying it a number of times I decide to check the error log, and find this...

Error building Player: Exception: android (invokation failed) ERROR: unknown error cmd:android list target -c stdout; stderr: Exception in thread "main" java.lang.NullPointerException at SDKMain.main(SDKMain.java:62)

Any ideas guys??

Thanks