No problem. You can only have a preference that doesn't get traded off with if it happens to never conflict with anything - for instance, my preference that there be a moon has yet to interact with any other preferences I could act towards fulfilling; the moon just goes on being regardless - or, if it's your only preference. Even if you have only one preference, there could be tradeoffs about instrumental subgoals. You might have to decide between a 50% chance of ten units of preference-fulfillment and a guarantee of five units, even if you'd really like to have both at once, even if the units are the only thing you care about.