MissingMethodException: GUI.Label

Unity Engine
1 
function OnGUI()
{
	GUI.Label(Rect(0, 0, Screen.width, Screen.height), "Hello World!");
}

Please, for the love of god, could somebody tell me why this simple piece of code which I’ve proof read again and again and again is giving me a MissingMethodException error. I should be able to handle crap like this, I’ve been doing Unity for months. Why?
WHYYYYYYYY???

EDIT: If you’re reading this, then perhaps you, like me, didn’t realize that you cannot name your script ‘GUI’.

2

I don’t see any obvious issues there (though I’m a C# guy myself). Can you post the entire error from Unity?