It depends on how high the spacer extends above the stem. Carbon steerer tubes need to be fully supported by the expander, in the area where the stem clamps the steerer. If the steerer extends far enough above the stem, the expander will not be supporting the clamped area of the steerer, leading to cracking and failure at the lower edge of the stem clamp. 5 - 10mm of spacer above the stem should be fine.