diff --git a/Lib/doctest.py b/Lib/doctest.py
index e80e74d..d6fb504 100644
--- a/Lib/doctest.py
+++ b/Lib/doctest.py
@@ -247,7 +247,8 @@
         return result
 
     def truncate(self, size=None):
-        StringIO.truncate(self, size)
+        self.seek(size)
+        StringIO.truncate(self)
 
 # Worst-case linear-time ellipsis matching.
 def _ellipsis_match(want, got):