diff --git a/Lib/turtle.py b/Lib/turtle.py
index 465d6e065d1..c9e88d9dd12 100644
--- a/Lib/turtle.py
+++ b/Lib/turtle.py
@@ -2594,7 +2594,7 @@ class RawTurtle(TPen, TNavigator):
         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)
diff --git a/Misc/NEWS b/Misc/NEWS
index b32698535d1..08b3c6fa919 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -30,6 +30,8 @@ Library
 - Issue #21044: tarfile.open() now handles fileobj with an integer 'name'
   attribute.  Based on patch by Martin Panter.
 
+- Issue #21867:  Prevent turtle crash due to invalid undo buffer size.
+
 - Issue #19076: Don't pass the redundant 'file' argument to self.error().
 
 - Issue #21942: Fixed source file viewing in pydoc's server mode on Windows.