column cannot be null

[The Little Guy]

I have this multi query:

LOCK TABLE categories WRITE;

SELECT @myRight := rgt FROM categories WHERE parent = '$parent';

UPDATE categories SET rgt = rgt + 2 WHERE rgt > @myRight;
UPDATE categories SET lft = lft + 2 WHERE lft > @myRight;

INSERT INTO categories(name, parent, lft, rgt) VALUES ('$value', $parent, @myRight + 1, @myRight + 2);

UNLOCK TABLES;

 

I am getting the following error when I do the insert:

 

#1048 - Column 'lft' cannot be null

 

What am I doing wrong?