Why are multiple numbers not workin in IN clause for DB2 z/OS

Question

So I have the following query:

SELECT * FROM SOMESCHEMA.SOMETABLE
WHERE ID IN (123);

Which does work as intended.

But

SELECT * FROM SOMESCHEMA.SOMETABLE
WHERE ID IN (123,456);

Does not work!

But "funny":

SELECT * FROM SOMESCHEMA.SOMETABLE
WHERE ID IN ('123','456');

Does return both entries I expected?

Why is that? And who had this great idea?!

ID is:

• TYPE_NAME: BIGINT
• COLUMN_SIZE: 19