To use a spacer above the stem, the amount of steerer tube protruding above the stem has to be enough to support the spacer (i.e, the spacer has to be able to "grab" the steerer tube), AND the top edge of the spacer has to be far enough above the top edge of the steerer tube to allow the top cap to seat properly (i.e., not bottom out against the top edge of the steerer tube as Hillrider has mentioned).