forcing null values into #variables?

["Aultman, Steven (SMAU)" <SMAU@CHEVRON.COM>]

SQR 3.0.13.1
Sybase 11.0.3

Is there a way to force a null value into a #variable?  I have code along
these lines:

Select
....
null &n

   move &n to #test_num

   (other processing that will assign a #test_num under certain conditions)


end-select

But when I look at #test_num, I see a zero.

I've tried a few other things, too, including searching the mailing list's
archives.
I'm trying to prevent the carry-over of calculated values from previous rows
by resetting them inside the select statement.

Thanks in advance for your consideration,
Steve Aultman
CITC