--- a/servo/components/style/matching.rs
+++ b/servo/components/style/matching.rs
@@ -1468,16 +1468,17 @@ pub trait MatchMethods : TElement {
     /// pseudo-element, compute the restyle damage used to determine which
     /// kind of layout or painting operations we'll need.
     fn compute_style_difference(&self,
                                 old_values: &ComputedValues,
                                 new_values: &Arc<ComputedValues>,
                                 pseudo: Option<&PseudoElement>)
                                 -> StyleDifference
     {
+        debug_assert!(pseudo.map_or(true, |p| p.is_eager()));
         if let Some(source) = self.existing_style_for_restyle_damage(old_values, pseudo) {
             return RestyleDamage::compute_style_difference(source, new_values)
         }
 
         let new_style_is_display_none =
             new_values.get_box().clone_display() == display::T::none;
         let old_style_is_display_none =
             old_values.get_box().clone_display() == display::T::none;