DEFAULT udf(x) does not return an error when UDF is dropped

--replace_result $UDF_EXAMPLE_SO UDF_EXAMPLE_LIB

eval CREATE FUNCTION metaphon RETURNS STRING SONAME "$UDF_EXAMPLE_SO";

SELECT METAPHON('Hello');

CREATE TABLE t1 (a VARCHAR(10), b VARCHAR(10) DEFAULT METAPHON(a));

INSERT INTO t1 (a) VALUES ('Hello');

SELECT * FROM t1;

DROP FUNCTION METAPHON;

# QQ: this should return an error

INSERT INTO t1 (a) VALUES ('Hello');

SELECT * FROM t1;

DROP TABLE t1;