diff -r c426da77695f Lib/test/test_warnings.py
--- a/Lib/test/test_warnings.py	Mon Dec 08 18:01:15 2014 -0500
+++ b/Lib/test/test_warnings.py	Tue Dec 09 13:43:27 2014 +0200
@@ -666,6 +666,15 @@ class _WarningsTests(BaseTest, unittest.
         finally:
             globals_dict['__file__'] = oldfile
 
+    def test_stderr_none(self):
+        rc, stdout, stderr = assert_python_ok("-c",
+            "import sys; sys.stderr = None; "
+            "import warnings; warnings.simplefilter('always'); "
+            "warnings.warn('Warning!')")
+        self.assertEqual(stdout, b'')
+        self.assertNotIn(b'Warning!', stderr)
+        self.assertNotIn(b'Error', stderr)
+
 
 class WarningsDisplayTests(BaseTest):
 
diff -r c426da77695f Lib/warnings.py
--- a/Lib/warnings.py	Mon Dec 08 18:01:15 2014 -0500
+++ b/Lib/warnings.py	Tue Dec 09 13:43:27 2014 +0200
@@ -11,6 +11,9 @@ def showwarning(message, category, filen
     """Hook to write a warning to a file; replace if you like."""
     if file is None:
         file = sys.stderr
+        if file is None:
+            # sys.stderr is None when ran with pythonw.exe - warnings get lost
+            return
     try:
         file.write(formatwarning(message, category, filename, lineno, line))
     except OSError: