why does Postgres raise a not-null constraint error on a serial column?

something strange is happening to my database. my understanding is that the SERIAL datatype is the Postgres equivalent of MySQL’s auto-increment and that omitting it from an INSERT clause will automatically fill it with the next value. I have a simple table of the following structure:

CREATE TABLE data.categories
(
    "categoryId" serial NOT NULL,
    "categoryName" character varying(256) NOT NULL,
    PRIMARY KEY ("categoryId")
)

I’m expecting to be able to add a record without having to specify the id, like this:

INSERT INTO data.categories("categoryName") VALUES('Undefined');

yet, it throws the following error and I don’t understand why, it’s not meant to happen:

ERROR:  null value in column "categoryId" violates not-null constraint
DETAIL:  Failing row contains (1, Undefined, null).
SQL state: 23502

any thoughts on what I’m doing wrong and why this is happening? is this a bug? every answer on stackoverflow "confirms" that my thinking is right and that using serial as the data type should solve the problem of manually managing ids.