diff --git a/Lib/unittest/mock.py b/Lib/unittest/mock.py
index 315b611..4203796 100644
--- a/Lib/unittest/mock.py
+++ b/Lib/unittest/mock.py
@@ -155,7 +155,7 @@ def _set_signature(mock, original, instance=False):
     skipfirst = isinstance(original, type)
     result = _get_signature_object(original, instance, skipfirst)
     if result is None:
-        return
+        return mock
     func, sig = result
     def checksig(*args, **kwargs):
         sig.bind(*args, **kwargs)
diff --git a/Lib/unittest/test/testmock/testhelpers.py b/Lib/unittest/test/testmock/testhelpers.py
index d5f9e7c..55cd1aa 100644
--- a/Lib/unittest/test/testmock/testhelpers.py
+++ b/Lib/unittest/test/testmock/testhelpers.py
@@ -1,3 +1,4 @@
+import time
 import unittest
 
 from unittest.mock import (
@@ -891,6 +892,18 @@ class TestCallList(unittest.TestCase):
         self.assertNotIn(call('fish'), mock.call_args_list)
         self.assertNotIn([call('fish')], mock.call_args_list)
 
+    def test_autospec_on_bound_builtin_function(self):
+        meth = six.create_bound_method(time.ctime, time.time())
+        self.assertIsInstance(meth(), str)
+        mocked = create_autospec(meth)
+
+        # no signature, so no spec to check against
+        mocked()
+        mocked.assert_called_once_with()
+        mocked.reset_mock()
+        mocked(4, 5, 6)
+        mocked.assert_called_once_with(4, 5, 6)
+
 
     def test_call_list_str(self):
         mock = Mock()