diff --git a/Lib/doctest.py b/Lib/doctest.py
index e02e73ed722f7e..02144cd544e80c 100644
--- a/Lib/doctest.py
+++ b/Lib/doctest.py
@@ -288,7 +288,8 @@ def getvalue(self):
         return result
 
     def truncate(self, size=None):
-        self.seek(size)
+        if size != None:
+            self.seek(size)
         StringIO.truncate(self)
 
 # Worst-case linear-time ellipsis matching.
diff --git a/Misc/ACKS b/Misc/ACKS
index 25542d01de695c..f530ae1f0a56b6 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -419,6 +419,7 @@ Lars Damerow
 Evan Dandrea
 Eric Daniel
 Scott David Daniels
+Aaron Davidson
 Derzsi Dániel
 Lawrence D'Anna
 Ben Darnell
diff --git a/Misc/NEWS.d/next/Library/2023-03-23-19-47-05.gh-issue-102967.gfIYkN.rst b/Misc/NEWS.d/next/Library/2023-03-23-19-47-05.gh-issue-102967.gfIYkN.rst
new file mode 100644
index 00000000000000..bcece939531869
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2023-03-23-19-47-05.gh-issue-102967.gfIYkN.rst
@@ -0,0 +1,2 @@
+A bug in :func:`doctest._SpoofOut.truncate` was causing None to be passed to :func:`StringIO.seek` when no size was given.
+A simple fix skips the seek call when no size is given so the buffer can be truncated from the current position.