diff --git a/Lib/fileinput.py b/Lib/fileinput.py
index 2f895e9..2e26b5b 100644
--- a/Lib/fileinput.py
+++ b/Lib/fileinput.py
@@ -142,6 +142,7 @@
         self._filelineno = 0
         self._file = None
         self._isstdin = 0
+        self._backupfilename = None
 
     def __del__(self):
         self.close()