Crash in Field_string::type_handler when calling procedures

CREATE TABLE t (f INT);

--delimiter $

CREATE TRIGGER tr AFTER INSERT ON t FOR EACH ROW

    FOR x IN (SELECT * FROM json_table(NULL, '$' COLUMNS(a CHAR(1) path '$.*')) tmp)  

    DO set @a=1; END FOR $

--delimiter ;

INSERT INTO t () values ();

# Cleanup

DROP TABLE t;