Problem with Array

[Cortana]

Im making a game which relies on multidimensional arrays. But Im getting a serious problem which can be reproduced with this code (in the Flash 9 preview)

var test:Array = new Array()
test.push(["a", "b"])
test.push(["c", "d"])
trace(test[0][0])	//	a
trace(test[1][0])	//	c
trace(test[0][-1])	//	undefined (perfectly acceptable)
trace(test[-1][0])	//	throws error : "TypeError: Error #1010: A term is undefined and has no properties."

How can i get around this error? It's annoying because I actually want it to return undefined!