diff --git a/Lib/asynchat.py b/Lib/asynchat.py
index 91f0bb2fd9c..fc6cabe7f25 100644
--- a/Lib/asynchat.py
+++ b/Lib/asynchat.py
@@ -99,6 +99,8 @@ def set_terminator(self, term):
         """
         if isinstance(term, str) and self.use_encoding:
             term = bytes(term, self.encoding)
+        elif isinstance(term, int) and term < 0:
+            raise ValueError('the number of received bytes must be positive')
         self.terminator = term
 
     def get_terminator(self):
diff --git a/Lib/test/test_asynchat.py b/Lib/test/test_asynchat.py
index f6b17f9f36e..9e305731ed3 100644
--- a/Lib/test/test_asynchat.py
+++ b/Lib/test/test_asynchat.py
@@ -311,5 +311,13 @@ def test_given_list(self):
         self.assertEqual(f.pop(), (0, None))
 
 
+class TestNotConnected(unittest.TestCase):
+    def test_disallow_negative_terminator(self):
+        # Issue #11259
+        client = asynchat.async_chat()
+        self.assertRaises(ValueError, client.set_terminator, -1)
+
+
+
 if __name__ == "__main__":
     unittest.main()
diff --git a/Misc/NEWS b/Misc/NEWS
index ce35e3eb39d..4fec3c35295 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -108,6 +108,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #11259: asynchat.async_chat().set_terminator() now raises a ValueError
+  if the number of received bytes is negative.
+
 - Issue #12523: asynchat.async_chat.push() now raises a TypeError if it doesn't
   get a bytes string