diff -r 82451c88b3c0 Lib/os.py
--- a/Lib/os.py	Thu Apr 11 19:18:22 2013 -0500
+++ b/Lib/os.py	Fri Apr 12 10:42:37 2013 +0200
@@ -644,7 +644,11 @@ class _Environ(MutableMapping):
         self._data = data
 
     def __getitem__(self, key):
-        value = self._data[self.encodekey(key)]
+        try:
+            value = self._data[self.encodekey(key)]
+        except KeyError:
+            # raise KeyError with the original key value
+            raise KeyError(key)
         return self.decodevalue(value)
 
     def __setitem__(self, key, value):
@@ -654,9 +658,13 @@ class _Environ(MutableMapping):
         self._data[key] = value
 
     def __delitem__(self, key):
-        key = self.encodekey(key)
-        self.unsetenv(key)
-        del self._data[key]
+        encodedkey = self.encodekey(key)
+        self.unsetenv(encodedkey)
+        try:
+            del self._data[encodedkey]
+        except KeyError:
+            # raise KeyError with the original key value
+            raise KeyError(key)
 
     def __iter__(self):
         for key in self._data:
diff -r 82451c88b3c0 Lib/test/test_os.py
--- a/Lib/test/test_os.py	Thu Apr 11 19:18:22 2013 -0500
+++ b/Lib/test/test_os.py	Fri Apr 12 10:42:37 2013 +0200
@@ -632,6 +632,24 @@ class EnvironTests(mapping_tests.BasicTe
             key = 'key='
             self.assertRaises(OSError, os.environ.__delitem__, key)
 
+    def test_key_type(self):
+        missing = 'missingkey'
+        self.assertNotIn(missing, os.environ)
+
+        try:
+            os.environ[missing]
+        except KeyError as err:
+            self.assertIs(err.args[0], missing)
+        else:
+            self.fail("KeyError not raised")
+
+        try:
+            del os.environ[missing]
+        except KeyError as err:
+            self.assertIs(err.args[0], missing)
+        else:
+            self.fail("KeyError not raised")
+
 class WalkTests(unittest.TestCase):
     """Tests for os.walk()."""