IIF function's result arguments against NULL Constant and NULL expression

IIF function returns exception if we pass both return arguments ( 2nd & 3rd arguments) as NULL constant; where as returns the correct output if we pass two NULL expressions/columns

Sample:

-- NULL constants 
Declare @DeptName VARCHAR(100) = 'IT'
SELECT IIF( @DeptName = 'IT' ,  NULL, NULL)
/*
Msg 8133, Level 16, State 1, Line 2
At least one of the result expressions in a CASE specification must be an expression other than the NULL constant.
*/

-- NULL expression/columns 
Declare @DeptName VARCHAR(100) = 'IT'
	,@SecondArgument int = NULL
	, @ThirdArgument INT = NULL
SELECT IIF( @DeptName = 'IT' ,  @SecondArgument, @ThirdArgument)

/*
Out put:
NULL
*/

NOTE: Be aware of this behaviour of IIF function