MySQL #1215 - Cannot add foreign key constraint

Question

I got problem on my table 2. I dont know what is the error all about, because it's not so specific. I've tried to google it, but wont work.

table 1

create table city(
city_id serial primary key,
city varchar(255),
province_id int,
foreign key(province_id) references province(province_id)
);

table 2

create table baranggay(
baranggay_id serial primary key,
baranggay varchar(255),
city_id int,
foreign key(city_id) references city(city_id)
);