diff -r c3cec0f77eff Lib/unittest/loader.py
--- a/Lib/unittest/loader.py	Tue Oct 20 18:22:36 2015 +0300
+++ b/Lib/unittest/loader.py	Tue Oct 20 22:07:34 2015 +0200
@@ -479,6 +479,8 @@
                     return tests, True
                 finally:
                     self._loading_packages.discard(name)
+        else:
+            return None, False
 
 
 defaultTestLoader = TestLoader()
diff -r c3cec0f77eff Lib/unittest/test/test_discovery.py
--- a/Lib/unittest/test/test_discovery.py	Tue Oct 20 18:22:36 2015 +0300
+++ b/Lib/unittest/test/test_discovery.py	Tue Oct 20 22:07:34 2015 +0200
@@ -90,6 +90,46 @@
                     ('test3', 'test4')])
         self.assertEqual(suite, expected)
 
+    def test_find_tests_socket(self):
+        # A socket is neither a directory nor a regular file.
+        # https://bugs.python.org/issue25320
+        loader = unittest.TestLoader()
+
+        original_listdir = os.listdir
+        def restore_listdir():
+            os.listdir = original_listdir
+        original_isfile = os.path.isfile
+        def restore_isfile():
+            os.path.isfile = original_isfile
+        original_isdir = os.path.isdir
+        def restore_isdir():
+            os.path.isdir = original_isdir
+
+        path_lists = [['socket']]
+        os.listdir = lambda path: path_lists.pop(0)
+        self.addCleanup(restore_listdir)
+
+        os.path.isdir = lambda path: False
+        self.addCleanup(restore_isdir)
+
+        os.path.isfile = lambda path: False
+        self.addCleanup(restore_isfile)
+
+        loader._get_module_from_name = lambda path: path + ' module'
+        orig_load_tests = loader.loadTestsFromModule
+        def loadTestsFromModule(module, pattern=None):
+            # This is where load_tests is called.
+            base = orig_load_tests(module, pattern=pattern)
+            return base + [module + ' tests']
+        loader.loadTestsFromModule = loadTestsFromModule
+        loader.suiteClass = lambda thing: thing
+
+        top_level = os.path.abspath('/foo')
+        loader._top_level_dir = top_level
+        suite = list(loader._find_tests(top_level, 'test*.py'))
+
+        self.assertEqual(suite, [])
+
     def test_find_tests_with_package(self):
         loader = unittest.TestLoader()