UPDATE using query; per-row function calling problem

I'm doing an UPDATE something like this:

    UPDATE
        slots
    SET
        a = 'a'
        ,b = (SELECT uuid_generate_v1())
    WHERE
        c = TRUE;

Each updated row in slots is getting the same value for b. Is there a
way of getting a per-row value from uuid_generate_v1() without doing a
PL loop?

Regards
Rory