modification of the column fails to check foreign key constraint

create table tab1 (

    t1id int unsigned not null auto_increment,

    t1val varchar(30) not null,

    constraint pkt1 primary key (t1id)

);

create table tab2 (

    t2id int unsigned not null auto_increment,

    t1id int unsigned,

    t2tval datetime,

    t2nval int,

    constraint pkt2 primary key (t2id)

);

alter table tab2 add index FkIdxt2t1id (t1id), add constraint Fkt2t1id foreign key (t1id) references tab1 (t1id);

alter table tab2 modify t1id int unsigned not null, modify t2tval timestamp(3), modify t2nval bigint;

Cannot change column 't1id': used in a foreign key constraint 'Fkt2t1id'

alter table tab2 modify t1id int unsigned not null;

alter table tab2 modify t2tval timestamp(3), modify t2nval bigint;