How to update a foreign key with random values from the referenced table?

In order to generate test data for a Postgres database, I want to update a foreign key in a table, let's say 'users' with random values from the referenced table, let's say 'shift'. How to do it? I have tried this way:

UPDATE users
SET id_shift = (
SELECT shift.id_shift
FROM shift
ORDER BY random()
LIMIT 1
);

I expected to have some users in one shift and others in another. But the result is that all users end up with the same shift.