Index: Lib/test/test_os.py
===================================================================
--- Lib/test/test_os.py	(revision 61521)
+++ Lib/test/test_os.py	(working copy)
@@ -509,6 +509,10 @@
     def test_chmod(self):
         self.assertRaises(WindowsError, os.utime, test_support.TESTFN, 0)
 
+class MiscTests(unittest.TestCase):
+    def test_strerror(self):
+        self.assertTrue(isinstance(os.strerror(0), str))
+
 if sys.platform != 'win32':
     class Win32ErrorTests(unittest.TestCase):
         pass
@@ -523,7 +527,8 @@
         MakedirTests,
         DevNullTests,
         URandomTests,
-        Win32ErrorTests
+        Win32ErrorTests,
+        MiscTests,
     )
 
 if __name__ == "__main__":
Index: Modules/posixmodule.c
===================================================================
--- Modules/posixmodule.c	(revision 61521)
+++ Modules/posixmodule.c	(working copy)
@@ -6710,7 +6710,6 @@
 }
 #endif /* unsetenv */
 
-#ifdef HAVE_STRERROR
 PyDoc_STRVAR(posix_strerror__doc__,
 "strerror(code) -> string\n\n\
 Translate an error code to a message string.");
@@ -6718,19 +6717,27 @@
 static PyObject *
 posix_strerror(PyObject *self, PyObject *args)
 {
-	int code;
+	int code, old_errno;
 	char *message;
 	if (!PyArg_ParseTuple(args, "i:strerror", &code))
 		return NULL;
+	old_errno = errno;
+	errno = 0;
 	message = strerror(code);
-	if (message == NULL) {
+	/* ISO C/POSIX do not allow NULL returns from strerror */
+	assert(message);
+	if (errno == EINVAL) {
 		PyErr_SetString(PyExc_ValueError,
 				"strerror() argument out of range");
 		return NULL;
 	}
+	else if (errno != 0)
+		/* Should not happen on ISO C/POSIX compliant platforms */
+		return posix_error();
+	errno = old_errno;
+
 	return PyString_FromString(message);
 }
-#endif /* strerror */
 
 
 #ifdef HAVE_SYS_WAIT_H
@@ -8510,9 +8517,7 @@
 #ifdef HAVE_UNSETENV
 	{"unsetenv",	posix_unsetenv, METH_VARARGS, posix_unsetenv__doc__},
 #endif
-#ifdef HAVE_STRERROR
 	{"strerror",	posix_strerror, METH_VARARGS, posix_strerror__doc__},
-#endif
 #ifdef HAVE_FCHDIR
 	{"fchdir",	posix_fchdir, METH_O, posix_fchdir__doc__},
 #endif