diff --git a/kgb/spies.py b/kgb/spies.py
index 30162dd367ac52483570f16b1a0ee46fda9bac24..45f299a580aab37d2584e0632c38334b09f6892f 100644
--- a/kgb/spies.py
+++ b/kgb/spies.py
@@ -310,7 +310,8 @@ class FunctionSpy(object):
                     self.owner = owner
 
         if (self.owner is not None and
-            self.func_type == self.TYPE_BOUND_METHOD):
+            self.func_type == self.TYPE_BOUND_METHOD and
+            not inspect.isclass(self.owner)):
             # Construct a replacement function for this method, and
             # re-assign it to the instance. We do this in order to
             # prevent two spies on the same function on two separate
diff --git a/kgb/tests/test_function_spy.py b/kgb/tests/test_function_spy.py
index 275875f55aae411cc8f4d8c39e6ee6046287f3fe..545ef14e631c1d67d68ecd0e22ace19f7d3fe277 100644
--- a/kgb/tests/test_function_spy.py
+++ b/kgb/tests/test_function_spy.py
@@ -79,10 +79,12 @@ class FunctionSpyTests(TestCase):
     def test_construction_with_call_fake_and_bound_method(self):
         """Testing FunctionSpy construction with call_fake and bound method"""
         obj = MathClass()
+        orig_method = obj.do_math
         spy = self.agency.spy_on(obj.do_math, call_fake=fake_do_math)
 
         self.assertTrue(hasattr(obj.do_math, 'spy'))
         self.assertIs(obj.do_math.spy, spy)
+        self.assertIsNot(obj.do_math, orig_method)
         self.assertFalse(hasattr(MathClass.do_math, 'spy'))
 
         self.assertEqual(spy.func, fake_do_math)
@@ -93,6 +95,7 @@ class FunctionSpyTests(TestCase):
     def test_construction_with_call_fake_and_unbound_method(self):
         """Testing FunctionSpy construction with call_fake and unbound method
         """
+        orig_method = MathClass.do_math
         spy = self.agency.spy_on(MathClass.do_math, call_fake=fake_do_math)
 
         self.assertTrue(hasattr(MathClass.do_math, 'spy'))
@@ -102,6 +105,15 @@ class FunctionSpyTests(TestCase):
         self.assertEqual(spy.func_type, spy.TYPE_UNBOUND_METHOD)
         self.assertEqual(spy.owner, MathClass)
 
+        if isinstance(orig_method, types.FunctionType):
+            # Python 3
+            self.assertIs(MathClass.do_math, orig_method)
+        elif isinstance(orig_method, types.MethodType):
+            # Python 2
+            self.assertIsNot(MathClass.do_math, orig_method)
+        else:
+            self.fail('Method has an unexpected type %r' % type(orig_method))
+
         obj = MathClass()
         self.assertTrue(hasattr(obj.do_math, 'spy'))
         self.assertIs(obj.do_math.spy, MathClass.do_math.spy)
@@ -112,11 +124,13 @@ class FunctionSpyTests(TestCase):
         def fake_class_do_math(cls, *args, **kwargs):
             return 42
 
+        orig_method = MathClass.class_do_math
         spy = self.agency.spy_on(MathClass.class_do_math,
                                  call_fake=fake_class_do_math)
 
         self.assertTrue(hasattr(MathClass.class_do_math, 'spy'))
         self.assertIs(MathClass.class_do_math.spy, spy)
+        self.assertIs(MathClass.class_do_math, orig_method)
 
         self.assertEqual(spy.func, fake_class_do_math)
         self.assertEqual(spy.orig_func, self.orig_class_do_math)