--- a/layout/style/ServoCSSRuleList.cpp
+++ b/layout/style/ServoCSSRuleList.cpp
@@ -128,16 +128,20 @@ ServoCSSRuleList::InsertRule(const nsASt
   return rv;
 }
 
 nsresult
 ServoCSSRuleList::DeleteRule(uint32_t aIndex)
 {
   nsresult rv = Servo_CssRules_DeleteRule(mRawRules, aIndex);
   if (!NS_FAILED(rv)) {
+    uintptr_t rule = mRules[aIndex];
+    if (rule > kMaxRuleType) {
+      CastToPtr(rule)->Release();
+    }
     mRules.RemoveElementAt(aIndex);
   }
   return rv;
 }
 
 ServoCSSRuleList::~ServoCSSRuleList()
 {
   EnumerateInstantiatedRules([](css::Rule* rule) { rule->Release(); });