--- a/layout/inspector/inDOMUtils.cpp
+++ b/layout/inspector/inDOMUtils.cpp
@@ -1364,20 +1364,29 @@ inDOMUtils::ClearPseudoClassLocks(nsIDOM
 
   return NS_OK;
 }
 
 NS_IMETHODIMP
 inDOMUtils::ParseStyleSheet(nsIDOMCSSStyleSheet *aSheet,
                             const nsAString& aInput)
 {
-  RefPtr<CSSStyleSheet> sheet = do_QueryObject(aSheet);
-  NS_ENSURE_ARG_POINTER(sheet);
+  RefPtr<CSSStyleSheet> geckoSheet = do_QueryObject(aSheet);
+  if (geckoSheet) {
+    NS_ENSURE_ARG_POINTER(geckoSheet);
+    return geckoSheet->ReparseSheet(aInput);
+  }
 
-  return sheet->ReparseSheet(aInput);
+  RefPtr<ServoStyleSheet> servoSheet = do_QueryObject(aSheet);
+  if (servoSheet) {
+    NS_ENSURE_ARG_POINTER(servoSheet);
+    return servoSheet->ReparseSheet(aInput);
+  }
+
+  return NS_ERROR_INVALID_POINTER;
 }
 
 NS_IMETHODIMP
 inDOMUtils::ScrollElementIntoView(nsIDOMElement *aElement)
 {
   nsCOMPtr<nsIContent> content = do_QueryInterface(aElement);
   NS_ENSURE_ARG_POINTER(content);