what is null

i'm trying to alter a table call it 'my_table' and i'm trying to set the
field 'my_id' to prevent null values from being inserted. when i run the
following:

ALTER TABLE my_table ALTER my_id SET NOT NULL;

i get:

ERROR: ALTER TABLE: Attribute "my_id" contains NULL values

so i want to find out which rows have null values so i run:

my=# select my_id from my_table where my_id=NULL;
my_id
---------
(0 rows)

so this would indicate to me that there are no rows in which my_id is NULL.
so what is preventing me from altering the table? or am i just doing
something wrong?

--
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