> Can we assume I know how to do argument checking?

I would probably question you at some point about a particular value of an argument and almost all candidates will get points for thinking out load that of course a NULL would get it crashing and that a simple check could stop it. Then it leads to discussion about how it should be handled. Sometimes a NULL means the program should cast an exception, other times it should just return without any side effect.

IMO the hate for whiteboard questions is really a symptom of poor interview technique.