IF...ELSE's Evaluation Of NULL Varchar Values

I have a stored procedure that accepts a parameter of type varchar(32). This parameter gets tested against NULL as part of an IF statement. The behavior I'm seeing is that this test never evaluates to TRUE, even in cases where NULL has been passed in as the value of the parameter.


CREATE PROCEDURE uspNewVital
@Description varchar(32),
@Type tinyint,
@DeviceType varchar(32),
@Dimension varchar(32),
@Unit varchar(32)
AS
IF (@Dimension != NULL AND @Unit != NULL)
BEGIN
RAISERROR ('You must specify either a dimension or a unit.', 15, 1) ;
RETURN 0 ;
END
IF NULL = @Dimension
BEGIN
DECLARE @UnitID AS int ;
SELECT @UnitID = ID FROM tblUnit WHERE Description=@Unit ;
IF @@ROWCOUNT = 0
BEGIN
RAISERROR ('Unit not found.', 15, 1) ;
RETURN 0 ;
END
INSERT INTO tblVitalType VALUES (@DeviceTypeID, @DatumID, NULL, @UnitID) ;
END
ELSE
BEGIN
DECLARE @DimensionID AS int ;
SELECT @DimensionID = ID FROM tblDimension WHERE Description=@Dimension ;
IF @@ROWCOUNT = 0
BEGIN
RAISERROR ('Dimension not found.', 15, 1) ;
RETURN 0 ;
END
INSERT INTO tblVitalType VALUES (@DeviceTypeID, @DatumID, @DimensionID, NULL) ;
END
GO


When I pass in a NULL for @Dimension, code execution still goes to the ELSE block.

At first I thought this might have something to do with the ANSI_NULLS option, but the database has this defaulting to OFF.


Any insights would be greatly appreciated. Thank you.