--- a/layout/style/ServoStyleSet.cpp
+++ b/layout/style/ServoStyleSet.cpp
@@ -638,16 +638,17 @@ ServoStyleSet::ReplaceSheets(SheetType a
 nsresult
 ServoStyleSet::InsertStyleSheetBefore(SheetType aType,
                                       ServoStyleSheet* aNewSheet,
                                       ServoStyleSheet* aReferenceSheet)
 {
   MOZ_ASSERT(aNewSheet);
   MOZ_ASSERT(aReferenceSheet);
   MOZ_ASSERT(aNewSheet->IsApplicable());
+  MOZ_ASSERT(aNewSheet != aReferenceSheet, "Can't place sheet before itself.");
 
   size_t idx = FindLocalIndexOfSheetOfType(aReferenceSheet, aType);
   if (idx == NO_RAW_INDEX) {
     return NS_ERROR_INVALID_ARG;
   }
   size_t referenceRawIndex = mSheets[aType][idx].rawIndex;
   size_t oldRawIndex = RemoveSheetOfType(aNewSheet, aType);
 

--- a/layout/style/StyleSetHandleInlines.h
+++ b/layout/style/StyleSetHandleInlines.h
@@ -183,17 +183,17 @@ StyleSetHandle::Ptr::ReplaceSheets(Sheet
 nsresult
 StyleSetHandle::Ptr::InsertStyleSheetBefore(SheetType aType,
                                 StyleSheet* aNewSheet,
                                 StyleSheet* aReferenceSheet)
 {
   FORWARD_CONCRETE(
     InsertStyleSheetBefore,
     (aType, aNewSheet->AsGecko(), aReferenceSheet->AsGecko()),
-    (aType, aReferenceSheet->AsServo(), aReferenceSheet->AsServo()));
+    (aType, aNewSheet->AsServo(), aReferenceSheet->AsServo()));
 }
 
 int32_t
 StyleSetHandle::Ptr::SheetCount(SheetType aType) const
 {
   FORWARD(SheetCount, (aType));
 }