diff -r 1e74350dd056 Lib/inspect.py
--- a/Lib/inspect.py	Tue Jun 17 22:27:46 2014 -0500
+++ b/Lib/inspect.py	Wed Jun 18 15:24:07 2014 +0300
@@ -1938,7 +1938,7 @@
     except AttributeError:
         pass
     else:
-        if sig is not None:
+        if sig is not None and isinstance(sig, Signature):
             return sig
 
     try:
diff -r 1e74350dd056 Lib/test/test_inspect.py
--- a/Lib/test/test_inspect.py	Tue Jun 17 22:27:46 2014 -0500
+++ b/Lib/test/test_inspect.py	Wed Jun 18 15:24:07 2014 +0300
@@ -3166,7 +3166,17 @@
             self.assertIn(module.__cached__, output)
         self.assertEqual(err, b'')
 
-
+    def test_custom_getattr(self):
+        class _Method:
+            def __init__(self, send, name):
+                self.__send = send
+                self.__name = name
+            def __getattr__(self, name):
+                return _Method(self.__send, "%s.%s" % (self.__name, name))
+            def __call__(self, a, b, c):
+                return self.__send(self.__name, a, b, c)
+        sig = inspect.signature(_Method("a", "b"))
+        self.assertEqual(str(sig), "(a, b, c)")
 
 
 def test_main():