diff -r 3ae2cd85a908 Lib/asynchat.py
--- a/Lib/asynchat.py	Sun Mar 09 11:18:16 2014 +0100
+++ b/Lib/asynchat.py	Sun Mar 09 08:46:30 2014 -0400
@@ -97,6 +97,8 @@
         "Set the input delimiter.  Can be a fixed string of any length, an integer, or None"
         if isinstance(term, str) and self.use_encoding:
             term = bytes(term, self.encoding)
+        elif isinstance(term, int) and term < 0:
+            raise ValueError('Negative terminator value is not allowed')
         self.terminator = term
 
     def get_terminator (self):
@@ -126,7 +128,7 @@
 
         while self.ac_in_buffer:
             lb = len(self.ac_in_buffer)
-            terminator = self.get_terminator()
+            terminator = self.terminator
             if not terminator:
                 # no terminator, collect it all
                 self.collect_incoming_data (self.ac_in_buffer)
@@ -137,7 +139,7 @@
                 if lb < n:
                     self.collect_incoming_data (self.ac_in_buffer)
                     self.ac_in_buffer = b''
-                    self.terminator = self.terminator - lb
+                    self.terminator -= lb
                 else:
                     self.collect_incoming_data (self.ac_in_buffer[:n])
                     self.ac_in_buffer = self.ac_in_buffer[n:]
diff -r 3ae2cd85a908 Lib/test/test_asynchat.py
--- a/Lib/test/test_asynchat.py	Sun Mar 09 11:18:16 2014 +0100
+++ b/Lib/test/test_asynchat.py	Sun Mar 09 08:46:30 2014 -0400
@@ -171,6 +171,9 @@
     def test_numeric_terminator2(self):
         self.numeric_terminator_check(6)
 
+    def test_disallow_negative_terminator(self):
+        self.assertRaises(ValueError, self.numeric_terminator_check, -42)
+
     def test_none_terminator(self):
         # Try reading a fixed number of bytes
         s, event = start_echo_server()