GREATEST(int_col,date_col) returns wrong results in a view

DROP TABLE IF EXISTS t1;

DROP VIEW IF EXISTS v1;

CREATE TABLE t1 (pk INT NOT NULL, d DATE NOT NULL);

CREATE VIEW v1 AS SELECT * FROM t1;

INSERT INTO t1 VALUES (1,'2018-06-22'),(2,'2018-07-11');

SELECT GREATEST(pk, d) FROM t1;

+-----------------+

| GREATEST(pk, d) |

+-----------------+

| 2018-06-22      |

| 2018-07-11      |

+-----------------+

SELECT GREATEST(pk, d) FROM v1;

+-----------------+

| GREATEST(pk, d) |

+-----------------+

| NULL            |

| NULL            |

+-----------------+