diff --git a/lib/python/common_extension.c b/lib/python/common_extension.c
index efc4b9e01..14f3292d9 100644
--- a/lib/python/common_extension.c
+++ b/lib/python/common_extension.c
@@ -51,9 +51,17 @@ static PyObject *PyObjectID_id(PyObject *self) {
                                     UNIQUE_ID_SIZE);
 }
 
+static PyObject *PyObjectID___reduce__(PyObjectID *self) {
+  PyErr_SetString(CommonError, "ObjectID objects cannot be serialized.");
+  return NULL;
+}
+
 static PyMethodDef PyObjectID_methods[] = {
     {"id", (PyCFunction) PyObjectID_id, METH_NOARGS,
      "Return the hash associated with this ObjectID"},
+    {"__reduce__", (PyCFunction) PyObjectID___reduce__, METH_NOARGS,
+     "Say how to pickle this ObjectID. This raises an exception to prevent"
+     "object IDs from being serialized."},
     {NULL} /* Sentinel */
 };
 
diff --git a/test/test.py b/test/test.py
index a40d2045c..359e8c030 100644
--- a/test/test.py
+++ b/test/test.py
@@ -1,5 +1,6 @@
 from __future__ import print_function
 
+import pickle
 import unittest
 
 import common
@@ -52,6 +53,23 @@ class TestObjectID(unittest.TestCase):
   def test_create_object_id(self):
     object_id = common.ObjectID(20 * "a")
 
+  def test_cannot_pickle_object_ids(self):
+    object_ids = [common.ObjectID(20 * chr(i)) for i in range(256)]
+    def f():
+      return object_ids
+    def g(val=object_ids):
+      return 1
+    def h():
+      x = object_ids[0]
+      return 1
+    # Make sure that object IDs cannot be pickled (including functions that
+    # close over object IDs).
+    self.assertRaises(Exception, lambda : pickling.dumps(object_ids[0]))
+    self.assertRaises(Exception, lambda : pickling.dumps(object_ids))
+    self.assertRaises(Exception, lambda : pickling.dumps(f))
+    self.assertRaises(Exception, lambda : pickling.dumps(g))
+    self.assertRaises(Exception, lambda : pickling.dumps(h))
+
 class TestTask(unittest.TestCase):
 
   def test_create_task(self):