diff -r 79e5bb0d9b8e Lib/test/test_os.py
--- a/Lib/test/test_os.py	Tue Feb 18 09:37:43 2014 +0100
+++ b/Lib/test/test_os.py	Tue Feb 18 12:51:16 2014 +0100
@@ -683,6 +683,10 @@ class EnvironTests(mapping_tests.BasicTe
         self.assertIs(cm.exception.args[0], missing)
         self.assertTrue(cm.exception.__suppress_context__)
 
+    def test_empty_key(self):
+        self.assertNotIn('', os.environ)
+        self.assertRaises(ValueError, os.environ.__setitem__, '', 'value')
+
 
 class WalkTests(unittest.TestCase):
     """Tests for os.walk()."""
diff -r 79e5bb0d9b8e Modules/posixmodule.c
--- a/Modules/posixmodule.c	Tue Feb 18 09:37:43 2014 +0100
+++ b/Modules/posixmodule.c	Tue Feb 18 12:51:16 2014 +0100
@@ -8890,13 +8890,21 @@ posix_putenv(PyObject *self, PyObject *a
     PyObject *os1, *os2;
     char *s1, *s2;
     char *newenv;
+    Py_ssize_t len;
 
     if (!PyArg_ParseTuple(args,
                           "O&O&:putenv",
                           PyUnicode_FSConverter, &os1,
                           PyUnicode_FSConverter, &os2))
         return NULL;
-    s1 = PyBytes_AsString(os1);
+
+    if (PyBytes_AsStringAndSize(os1, &s1, &len) < 0)
+        goto error;
+    if (len == 0) {
+        PyErr_SetString(PyExc_ValueError, "key is empty");
+        goto error;
+    }
+
     s2 = PyBytes_AsString(os2);
 
     newstr = PyBytes_FromFormat("%s=%s", s1, s2);