Null check in ActionScript/Flex

This is a language question rather than an IntelliJ IDEA question. I'm new to Flex development. In my code, if I enter the syntax:

if (_myVariable == null)

IntelliJ IDEA's inspection warns me that "Comparison may cause unexpected type coercoin". But it does not provide a quick fix. What is the proper way to do null and not null checks in ActionScript/JavaScript?