diff --git a/dom/base/DocumentOrShadowRoot.cpp b/dom/base/DocumentOrShadowRoot.cpp
index edf4b114444..71bfe7a1517 100644
--- a/dom/base/DocumentOrShadowRoot.cpp
+++ b/dom/base/DocumentOrShadowRoot.cpp
@@ -113,6 +113,10 @@ void DocumentOrShadowRoot::RemoveSheetFromStylesIfApplicable(
 void DocumentOrShadowRoot::OnSetAdoptedStyleSheets(StyleSheet& aSheet,
                                                    uint32_t aIndex,
                                                    ErrorResult& aRv) {
+  if (aIndex > mAdoptedStyleSheets.Length()) [[unlikely]] {
+    MOZ_ASSERT_UNREACHABLE("Out of sync proxy");
+    return;
+  }
   Document& doc = *AsNode().OwnerDoc();
   // 1. If value’s constructed flag is not set, or its constructor document is
   // not equal to this DocumentOrShadowRoot's node document, throw a
@@ -165,7 +169,10 @@ void DocumentOrShadowRoot::OnSetAdoptedStyleSheets(StyleSheet& aSheet,
 void DocumentOrShadowRoot::OnDeleteAdoptedStyleSheets(StyleSheet& aSheet,
                                                       uint32_t aIndex,
                                                       ErrorResult&) {
-  MOZ_ASSERT(mAdoptedStyleSheets.ElementAt(aIndex) == &aSheet);
+  if (mAdoptedStyleSheets.ElementAt(aIndex) != &aSheet) [[unlikely]] {
+    MOZ_ASSERT_UNREACHABLE("Out of sync proxy");
+    return;
+  }
   mAdoptedStyleSheets.RemoveElementAt(aIndex);
   auto existingIndex = mAdoptedStyleSheets.LastIndexOf(&aSheet);
   if (existingIndex != mAdoptedStyleSheets.NoIndex && existingIndex >= aIndex) {