Merge into does not work

[Shaozhong SHI <shishaozhong@xxxxxxxxx>]

CREATE TABLE Stock(item_id int UNIQUE, balance int);

INSERT INTO Stock VALUES (10, 2200);
INSERT INTO Stock VALUES (20, 1900);

CREATE TABLE Buy(item_id int, volume int);

INSERT INTO Buy values(10, 1000);
INSERT INTO Buy values(30, 300);
MERGE INTO Stock USING Buy ON Stock.item_id = Buy.item_id
 WHEN MATCHED THEN UPDATE SET balance = balance + Buy.volume
 WHEN NOT MATCHED THEN INSERT VALUES (Buy.item_id, Buy.volume);
I am using Postgres 9.6.
Regards,
David