SQL stored procedure variable null checking

Below is part of a stored procedure that I have, it would be of great help if you can point me why is that even when `@DefaultID` is null after executing the select statement it does not enter the if condition, is there are different method I have to follow to assign the value to `@DefaultID` for it to be available for null checking. Thank you in advance. ``` DECLARE @DefaultID varchar(25) DECLARE @ISDefault varchar(1) SELECT @DefaultID = (SELECT TABLE1.DEFID as DEFID FROM TABLE1 WHERE TABLE1.ID = '123') IF (@DefaultID = '') SET @ISDefault = '1' ELSE SET @ISDefault = '0' ```