--- a/editor/libeditor/HTMLEditRules.cpp
+++ b/editor/libeditor/HTMLEditRules.cpp
@@ -7859,17 +7859,20 @@ HTMLEditRules::PopListItem(nsIDOMNode* a
   nsCOMPtr<Element> listItem = do_QueryInterface(aListItem);
   // check parms
   NS_ENSURE_TRUE(listItem && aOutOfList, NS_ERROR_NULL_POINTER);
 
   // init out params
   *aOutOfList = false;
 
   nsCOMPtr<nsINode> curParent = listItem->GetParentNode();
-  int32_t offset = curParent ? curParent->IndexOf(listItem) : -1;
+  if (NS_WARN_IF(!curParent)) {
+    return NS_ERROR_FAILURE;
+  }
+  int32_t offset = curParent->IndexOf(listItem);
 
   if (!HTMLEditUtils::IsListItem(listItem)) {
     return NS_ERROR_FAILURE;
   }
 
   // if it's first or last list item, don't need to split the list
   // otherwise we do.
   nsCOMPtr<nsINode> curParPar = curParent->GetParentNode();