diff --git a/src/zope/i18nmessageid/_zope_i18nmessageid_message.c b/src/zope/i18nmessageid/_zope_i18nmessageid_message.c
index 66cd93d..450b9fe 100644
--- a/src/zope/i18nmessageid/_zope_i18nmessageid_message.c
+++ b/src/zope/i18nmessageid/_zope_i18nmessageid_message.c
@@ -100,8 +100,13 @@ Message_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
   if (default_ != NULL)
     self->default_ = default_;
 
-  if (mapping != NULL)
-    self->mapping = mapping;
+  if (mapping == Py_None) {
+    self->mapping = Py_None;
+    Py_INCREF(Py_None);
+  } else if (mapping != NULL) {
+    self->mapping = PyDictProxy_New(mapping);
+  } else {
+  }
 
   if (value_plural != NULL)
     self->value_plural = value_plural;
@@ -172,15 +177,27 @@ Message_dealloc(Message *self)
 static PyObject *
 Message_reduce(Message *self)
 {
-  PyObject *value, *result;
+  PyObject *value, *mapping, *result;
   value = PyObject_CallFunctionObjArgs((PyObject *)&PyUnicode_Type, self, NULL);
   if (value == NULL)
     return NULL;
+  if (self->mapping == NULL) {
+    mapping = Py_None;
+  }
+  else if (self->mapping == Py_None) {
+    mapping = Py_None;
+  } else {
+    mapping = PyObject_CallFunctionObjArgs(
+        (PyObject *)&PyDict_Type, self->mapping, NULL);
+    if (mapping == NULL) {
+        return NULL;
+    }
+  }
   result = Py_BuildValue("(O(OOOOOOO))", Py_TYPE(&(self->base)),
              value,
              self->domain ? self->domain : Py_None,
              self->default_ ? self->default_ : Py_None,
-             self->mapping ? self->mapping : Py_None,
+             mapping,
              self->value_plural ? self->value_plural : Py_None,
              self->default_plural ? self->default_plural : Py_None,
              self->number ? self->number : Py_None);
diff --git a/src/zope/i18nmessageid/message.py b/src/zope/i18nmessageid/message.py
index bd06e8d..122c14d 100644
--- a/src/zope/i18nmessageid/message.py
+++ b/src/zope/i18nmessageid/message.py
@@ -13,6 +13,8 @@
 ##############################################################################
 """I18n Messages and factories.
 """
+import types
+
 
 __docformat__ = "reStructuredText"
 _marker = object()
@@ -54,8 +56,10 @@ def __new__(cls, ustr, domain=_marker, default=_marker, mapping=_marker,
             self.domain = domain
         if default is not _marker:
             self.default = default
-        if mapping is not _marker:
-            self.mapping = mapping
+        if mapping is None:
+            self.mapping = None
+        elif mapping is not _marker:
+            self.mapping = types.MappingProxyType(mapping)
         if msgid_plural is not _marker:
             self.msgid_plural = msgid_plural
         if default_plural is not _marker:
@@ -81,9 +85,17 @@ def __setattr__(self, key, value):
             return str.__setattr__(self, key, value)
 
     def __getstate__(self):
+        # types.MappingProxyType is not picklable
+        mapping = None if self.mapping is None else dict(self.mapping)
         return (
-            str(self), self.domain, self.default, self.mapping,
-            self.msgid_plural, self.default_plural, self.number)
+            str(self),
+            self.domain,
+            self.default,
+            mapping,
+            self.msgid_plural,
+            self.default_plural,
+            self.number,
+        )
 
     def __reduce__(self):
         return self.__class__, self.__getstate__()
diff --git a/src/zope/i18nmessageid/tests.py b/src/zope/i18nmessageid/tests.py
index 1b2bbee..30ba48a 100644
--- a/src/zope/i18nmessageid/tests.py
+++ b/src/zope/i18nmessageid/tests.py
@@ -53,9 +53,20 @@ def test_values(self):
         self.assertEqual(message.msgid_plural, 'testings')
         self.assertEqual(message.default_plural, 'defaults')
         self.assertEqual(message.number, 2)
+
+        with self.assertRaises(TypeError):
+            message.mapping['key'] = 'new value'
+
         if self._TEST_READONLY:
             self.assertTrue(message._readonly)
 
+    def test_mapping_is_readonly(self):
+        mapping = {'key': 'value'}
+        message = self._makeOne('testing', 'domain', mapping=mapping)
+
+        with self.assertRaises(TypeError):
+            message.mapping['key'] = 'new value'
+
     def test_values_without_defaults(self):
         mapping = {'key': 'value'}
         message = self._makeOne(
@@ -206,6 +217,16 @@ def test_unknown_immutable(self):
         with self.assertRaises((TypeError, AttributeError)):
             message.unknown = 'unknown'
 
+    def test___reduce___wo_values(self):
+        message = self._makeOne('testing')
+        klass, state = message.__reduce__()
+        self.assertTrue(klass is self._getTargetClass())
+        self.assertTrue(message.mapping is None)
+        self.assertEqual(
+            state,
+            ('testing', None, None, None, None, None, None)
+        )
+
     def test___reduce__(self):
         mapping = {'key': 'value'}
         source = self._makeOne('testing')