SQL syntax error when creating a stored procedure in MySQL

I have a hard time locating an error when trying to create a stored procedure in mysql.

If I run every single line of the procedure independently, everything works just fine.

CREATE PROCEDURE cms_proc_add_child 
(
    param_parent_id INT, param_name CHAR(255),
    param_content_type CHAR(255)
)
BEGIN
    SELECT @child_left := rgt FROM cms_tree WHERE id = param_parent_id;
    UPDATE cms_tree SET rgt = rgt+2 WHERE rgt >= @child_left;
    UPDATE cms_tree SET lft = lft+2 WHERE lft >= @child_left;
    INSERT INTO cms_tree (name, lft, rgt, content_type) VALUES 
    (
        param_name,
        @child_left,
        @child_left+1,
        param_content_type
    );
END

I get the following (helpful) error:

ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '' at line 3

I just don't know where to start debugging, as every single one of these lines is correct.

Do you have any tips?