UPDATE Syntax Error

I _think_ today's the worst cold day and my head will clear from now on.
In the meantime, I'm not seeing why an UPDATE statement does not return the
number of updated rows.

The idea is to replace NULL values in the units column with 'mg/L':

jerrittwq=# update monitor set units = 'mg/L' where units = NULL;
UPDATE 0

and there should be several thousands of updated rows. I've also tried
specifying the units attribute as '' and ' ' with no better results. Should
be a simple statement and I'm not seeing why it fails.

FWIW, I tried specifying an alias for the table, but that barfed for a
different reason:

jerrittwq=# update monitor as m set m.units = 'mg/L' where m.units = NULL;
ERROR: column "m" of relation "monitor" does not exist
LINE 1: update monitor as m set m.units = 'mg/L' where m.units = NUL...

Clue stick, please,

Rich