--- configparser.py	Wed Aug  3 16:36:11 2011
+++ configparser.py	Wed Aug  3 16:36:53 2011
@@ -625,6 +625,8 @@
         self._empty_lines_in_values = empty_lines_in_values
         if interpolation is _UNSET:
             self._interpolation = self._DEFAULT_INTERPOLATION
+        elif interpolation is None:
+            self._interpolation = Interpolation()
         else:
             self._interpolation = interpolation
         self.default_section=default_section