How to annotate a null-checking function correctly?

Hello,

I have a function called IsEmpty which checks for null or empty objects.

A sample call can look like this:

if(IsEmpty(value)) Foo();

RS marks value as a possible NullReferenceException.

The question is: how should I annotate the method so that RS recognizes this method as one that checks for nulls?

I'm using RS build 804 with VS 2008.

Thanks in advance!
shovavnik