To me a reasonable utility function has to have a degree of self-consistency. A reasonable utility function wouldn't value both doing and undoing the same action simultaneously.

If an entity is using a utility function to determine its actions, then for every action the entity can perform, its utility function must be able to determine a utility value which then determines whether the entity does the action or not. If the utility function does not return a value, then the entity still has to act or not act, so the entity still has a utility function for that action (non-action).

The purpose of a utility function is to inform the entity so it seeks to perform actions that result in greater utility. A utility function that is self-contradictory defeats the whole purpose of a utility function. While an arbitrary utility function can in principle occur, an intelligent entity with a self-contradictory utility function would achieve greater utility by modifying its utility function until it was less self-contradictory.

It is probably not possible to have a utility function that is both complete (in that it returns a utility for each action the entity can perform) and consistent (that it returns a single value for the utility of each action the entity can perform) except for very simple entities. An entity complex enough to instantiate arithmetic is complex enough to invoke Gödel's theorem. An entity can substitute a random choice when its utility function does not return a value, but that will result in sub-optimal results.

In the example that FAWS used, a utility function that seeks to annoy me as much as possible, is inconsistent with the entity being an omnipotent AI that can simulate something as complex as me, an entity which can instantiate arithmetic. The only annoyance the AI has caused me is a -1 karma, which to me is less than a single dust mote in the eye.