diff -r b7c0137cccbe Lib/test/test_xdrlib.py
--- a/Lib/test/test_xdrlib.py	Thu Mar 26 23:50:57 2015 +0100
+++ b/Lib/test/test_xdrlib.py	Fri Mar 27 13:59:18 2015 +0100
@@ -51,6 +51,13 @@
         up.done()
         self.assertRaises(EOFError, up.unpack_uint)
 
+    def test_pack_string(self):
+        p = xdrlib.Packer()
+
+        with self.assertRaises(TypeError):
+            p.pack_string('hello world')
+        self.assertFalse(p.get_buffer())
+
 class ConversionErrorTest(unittest.TestCase):
 
     def setUp(self):
diff -r b7c0137cccbe Lib/xdrlib.py
--- a/Lib/xdrlib.py	Thu Mar 26 23:50:57 2015 +0100
+++ b/Lib/xdrlib.py	Fri Mar 27 13:59:18 2015 +0100
@@ -105,7 +105,12 @@
     def pack_string(self, s):
         n = len(s)
         self.pack_uint(n)
-        self.pack_fstring(n, s)
+        try:
+            self.pack_fstring(n, s)
+        except TypeError as e:
+            self.__buf.seek(self.__buf.tell() - 4)
+            self.__buf.truncate()
+            raise e
 
     pack_opaque = pack_string
     pack_bytes = pack_string