--- a/layout/style/nsStyleSet.cpp
+++ b/layout/style/nsStyleSet.cpp
@@ -1736,20 +1736,17 @@ SkipAnimationRules(nsRuleNode* aRuleNode
   return ruleNode;
 }
 
 already_AddRefed<nsStyleContext>
 nsStyleSet::ReparentStyleContext(nsStyleContext* aStyleContext,
                                  nsStyleContext* aNewParentContext,
                                  Element* aElement)
 {
-  if (!aStyleContext) {
-    NS_NOTREACHED("must have style context");
-    return nullptr;
-  }
+  MOZ_ASSERT(aStyleContext, "aStyleContext must not be null");
 
   // This short-circuit is OK because we don't call TryStartingTransition
   // during style reresolution if the style context pointer hasn't changed.
   if (aStyleContext->GetParent() == aNewParentContext) {
     nsRefPtr<nsStyleContext> ret = aStyleContext;
     return ret.forget();
   }