diff --git a/Lib/StringIO.py b/Lib/StringIO.py
index 334bf853a2c..6952b1a999e 100644
--- a/Lib/StringIO.py
+++ b/Lib/StringIO.py
@@ -129,6 +129,8 @@ def write(self, s):
 				self.buflist = []
 			self.buflist = [self.buf[:self.pos], s, self.buf[newpos:]]
 			self.buf = ''
+			if newpos > self.len:
+				self.len = newpos
 		else:
 			self.buflist.append(s)
 			self.len = newpos