函数从update语句成功调用，但从select语句调用时出错

CREATE TABLE plch_test
(
   x   NUMBER
 , y   VARCHAR2 (3)
);

BEGIN
   INSERT INTO plch_test
        VALUES (1, 'NO');

   INSERT INTO plch_test
        VALUES (2, NULL);

   COMMIT;
END;
/

CREATE OR REPLACE FUNCTION silly_function (p_x NUMBER)
   RETURN VARCHAR2
IS
   l_y   plch_test.y%TYPE;
BEGIN
   SELECT t.y
     INTO l_y
     FROM plch_test t
    WHERE t.x = silly_function.p_x;

   RETURN l_y;
END;
CREATE TABLE plch_test
(
   x   NUMBER
 , y   V