Index: Modules/posixmodule.c
===================================================================
--- Modules/posixmodule.c	(revision 67313)
+++ Modules/posixmodule.c	(working copy)
@@ -2087,7 +2087,6 @@
 		if (PyArg_ParseTuple(args, "U:listdir", &po)) {
 			WIN32_FIND_DATAW wFileData;
 			Py_UNICODE *wnamebuf;
-			Py_UNICODE wch;
 			/* Overallocate for \\*.*\0 */
 			len = PyUnicode_GET_SIZE(po);
 			wnamebuf = malloc((len + 5) * sizeof(wchar_t));
@@ -2096,9 +2095,11 @@
 				return NULL;
 			}
 			wcscpy(wnamebuf, PyUnicode_AS_UNICODE(po));
-			wch = len > 0 ? wnamebuf[len-1] : '\0';
-			if (wch != L'/' && wch != L'\\' && wch != L':')
-				wnamebuf[len++] = L'\\';
+			if (len > 0) {
+				Py_UNICODE wch = wnamebuf[len-1];
+				if (wch != L'/' && wch != L'\\' && wch != L':')
+					wnamebuf[len++] = L'\\';
+			}
 			wcscpy(wnamebuf + len, L"*.*");
 			if ((d = PyList_New(0)) == NULL) {
 				free(wnamebuf);
Index: Lib/test/test_unicode_file.py
===================================================================
--- Lib/test/test_unicode_file.py	(revision 67313)
+++ Lib/test/test_unicode_file.py	(working copy)
@@ -161,6 +161,18 @@
                                TESTFN_UNICODE_UNENCODEABLE+ext,
                                False)
 
+    def test_listdir(self):
+        list1 = os.listdir('')
+        list2 = os.listdir(b'')
+        # Most files should be identical, but do not try
+        # too hard on non-ascii names.
+        for name1, name2 in zip(list1, list2):
+            try:
+                name1 = name1.encode('ascii')
+            except UnicodeError:
+                continue
+            self.assertEqual(name1, name2)
+
 def test_main():
     run_unittest(__name__)