diff -r 1bebfe9846f2 Lib/turtle.py
--- a/Lib/turtle.py	Wed Jun 25 09:10:56 2014 -0700
+++ b/Lib/turtle.py	Wed Jun 25 09:13:13 2014 -0700
@@ -2594,7 +2594,7 @@
         Example (for a Turtle instance named turtle):
         >>> turtle.setundobuffer(42)
         """
-        if size is None:
+        if size is None or size <= 0:
             self.undobuffer = None
         else:
             self.undobuffer = Tbuffer(size)