SET NEW = (SELECT ...) in a BEFORE INSERT trigger fails with ER_OPERAND_COLUMNS (1241) on a single-column table

CREATE TABLE t (a INT);                                                                                                                                                                                          

CREATE TRIGGER tr BEFORE INSERT ON t FOR EACH ROW SET NEW = (SELECT 5);                                                                                                                                          

INSERT INTO t VALUES (1);  -- ERROR 1241 (21000): Operand should contain 1 column(s); expected: row 5 inserted 

13.2.0-opt>CREATE TABLE t (a INT);                                                                                                                                                                                          

Query OK, 0 rows affected (0.008 sec)

13.2.0-opt>CREATE TRIGGER tr BEFORE INSERT ON t FOR EACH ROW SET NEW = (SELECT 5);                                                                                                                                          

Query OK, 0 rows affected (0.005 sec)

13.2.0-opt>INSERT INTO t VALUES (1); 

ERROR 1241 (21000): Operand should contain 1 column(s)

13.2.0-opt>