--- a/mailnews/base/search/src/nsMsgFilterService.cpp
+++ b/mailnews/base/search/src/nsMsgFilterService.cpp
@@ -1003,16 +1003,17 @@ nsresult nsMsgApplyFiltersToMessages::Ru
   return AdvanceToNextFolder();
 }
 
 NS_IMETHODIMP nsMsgFilterService::ApplyFilters(nsMsgFilterTypeType aFilterType,
                                                nsIArray *aMsgHdrList,
                                                nsIMsgFolder *aFolder,
                                                nsIMsgWindow *aMsgWindow)
 {
+  NS_ENSURE_ARG_POINTER(aFolder);
   nsCOMPtr<nsIMsgFilterList>    filterList;
   nsresult rv = aFolder->GetFilterList(aMsgWindow, getter_AddRefs(filterList));
   NS_ENSURE_SUCCESS(rv, rv);
 
   nsCOMPtr<nsISupportsArray>    folderList;
   rv = NS_NewISupportsArray( getter_AddRefs(folderList) );
   NS_ENSURE_SUCCESS(rv, rv);
   folderList->AppendElement(aFolder);