diff --git a/mongoengine/queryset/base.py b/mongoengine/queryset/base.py
index cb48f6ca..89a5e5fb 100644
--- a/mongoengine/queryset/base.py
+++ b/mongoengine/queryset/base.py
@@ -459,6 +459,8 @@ class BaseQuerySet(object):
                 return result
             elif result:
                 return result['n']
+        except pymongo.errors.DuplicateKeyError, err:
+            raise NotUniqueError(u'Update failed (%s)' % unicode(err))
         except pymongo.errors.OperationFailure, err:
             if unicode(err) == u'multi not coded yet':
                 message = u'update() method requires MongoDB 1.1.3+'
diff --git a/tests/document/instance.py b/tests/document/instance.py
index 6c4da162..afb27e0d 100644
--- a/tests/document/instance.py
+++ b/tests/document/instance.py
@@ -15,7 +15,7 @@ from tests.fixtures import (PickleEmbedded, PickleTest, PickleSignalsTest,
 
 from mongoengine import *
 from mongoengine.errors import (NotRegistered, InvalidDocumentError,
-                                InvalidQueryError)
+                                InvalidQueryError, NotUniqueError)
 from mongoengine.queryset import NULLIFY, Q
 from mongoengine.connection import get_db
 from mongoengine.base import get_document
@@ -990,6 +990,16 @@ class InstanceTest(unittest.TestCase):
 
         self.assertRaises(InvalidQueryError, update_no_op_raises)
 
+    def test_update_unique_field(self):
+        class Doc(Document):
+            name = StringField(unique=True)
+
+        doc1 = Doc(name="first").save()
+        doc2 = Doc(name="second").save()
+
+        self.assertRaises(NotUniqueError, lambda:
+                          doc2.update(set__name=doc1.name))
+
     def test_embedded_update(self):
         """
         Test update on `EmbeddedDocumentField` fields