diff --git a/components/style/matching.rs b/components/style/matching.rs
index 6dc335de1dc..eb88914f8ed 100644
--- a/components/style/matching.rs
+++ b/components/style/matching.rs
@@ -357,6 +357,7 @@ trait PrivateMatchMethods: TElement {
             // below like a lazy pseudo.
             let only_default_rules = context.shared.traversal_flags.for_default_styles();
             if pseudo.is_eager() && !only_default_rules {
+                debug_assert!(pseudo.is_before_or_after());
                 let parent = self.parent_element().unwrap();
                 if !parent.may_have_animations() ||
                    primary_style.rules.get_animation_rules().is_empty() {