sql_mode=traditional: assertion about NULL fails in Item_func_round::date_op

USE test;

CREATE TABLE t (a TIMESTAMP,b TIMESTAMP);  # (ENGINE=InnoDB) 

INSERT INTO t VALUES (0,0);

SET SQL_MODE='TRADITIONAL';

SELECT TRUNCATE(a,b) AS c FROM t;

SET sql_mode='';

CREATE TABLE t5 (c TIMESTAMP KEY,c2 INT NOT NULL,c3 CHAR(1) NOT NULL);

INSERT INTO t5 (c) VALUES (1);

SET sql_mode=traditional;

SELECT ROUND (c,c2),TRUNCATE (c,c2) FROM t5;