diff --git a/Lib/formatter.py b/Lib/formatter.py
index 507ed8a..25dbe73 100644
--- a/Lib/formatter.py
+++ b/Lib/formatter.py
@@ -265,9 +265,9 @@
 
 
 class NullWriter:
-    """Minimal writer interface to use in testing.
-    """
+    """Minimal writer interface to use in testing & inheritance."""
     def __init__(self): pass
+    def flush(self): pass
     def new_alignment(self, align): pass
     def new_font(self, font): pass
     def new_margin(self, margin, level): pass