diff --git a/docs/changelog.rst b/docs/changelog.rst
index e3f3ecd1..d0af955e 100644
--- a/docs/changelog.rst
+++ b/docs/changelog.rst
@@ -5,9 +5,13 @@ Changelog
 
 Changes in 0.9.X - DEV
 ======================
+<<<<<<< HEAD
 - OperationError: Shard Keys are immutable. Tried to update id even though the document is not yet saved #771
 - with_limit_and_skip for count should default like in pymongo #759
 - Fix storing value of precision attribute in DecimalField #787
+=======
+- Set attribute to None does not work (at least for fields with default values) #734
+>>>>>>> bdbd495a9e778d12157b789a74e0e00a303b5efd
 - Querying by a field defined in a subclass raises InvalidQueryError #744
 - Add Support For MongoDB 2.6.X's maxTimeMS #778
 - abstract shouldn't be inherited in EmbeddedDocument # 789
diff --git a/mongoengine/base/fields.py b/mongoengine/base/fields.py
index 5bb9c7ac..2747dde1 100644
--- a/mongoengine/base/fields.py
+++ b/mongoengine/base/fields.py
@@ -37,7 +37,7 @@ class BaseField(object):
     def __init__(self, db_field=None, name=None, required=False, default=None,
                  unique=False, unique_with=None, primary_key=False,
                  validation=None, choices=None, verbose_name=None,
-                 help_text=None):
+                 help_text=None, null=False):
         """
         :param db_field: The database field to store this field in
             (defaults to the name of the field)
@@ -60,6 +60,8 @@ class BaseField(object):
             model forms from the document model.
         :param help_text: (optional) The help text for this field and is often
             used when generating model forms from the document model.
+        :param null: (optional) Is the field value can be null. If no and there is a default value
+            then the default value is set
         """
         self.db_field = (db_field or name) if not primary_key else '_id'
 
@@ -75,6 +77,7 @@ class BaseField(object):
         self.choices = choices
         self.verbose_name = verbose_name
         self.help_text = help_text
+        self.null = null
 
         # Adjust the appropriate creation counter, and save our local copy.
         if self.db_field == '_id':
@@ -100,10 +103,13 @@ class BaseField(object):
 
         # If setting to None and theres a default
         # Then set the value to the default value
-        if value is None and self.default is not None:
-            value = self.default
-            if callable(value):
-                value = value()
+        if value is None:
+            if self.null:
+                value = None
+            elif self.default is not None:
+                value = self.default
+                if callable(value):
+                    value = value()
 
         if instance._initialised:
             try:
diff --git a/tests/document/instance.py b/tests/document/instance.py
index b7733548..5360ba36 100644
--- a/tests/document/instance.py
+++ b/tests/document/instance.py
@@ -57,7 +57,9 @@ class InstanceTest(unittest.TestCase):
             self.db.drop_collection(collection)
 
     def assertDbEqual(self, docs):
-        self.assertEqual(list(self.Person._get_collection().find().sort("id")), sorted(docs, key=lambda doc: doc["_id"]))
+        self.assertEqual(
+            list(self.Person._get_collection().find().sort("id")),
+            sorted(docs, key=lambda doc: doc["_id"]))
 
     def test_capped_collection(self):
         """Ensure that capped collections work properly.
@@ -468,7 +470,7 @@ class InstanceTest(unittest.TestCase):
             f.reload()
         except Foo.DoesNotExist:
             pass
-        except Exception as ex:
+        except Exception:
             self.assertFalse("Threw wrong exception")
 
     def test_dictionary_access(self):
@@ -2613,7 +2615,9 @@ class InstanceTest(unittest.TestCase):
         system.save()
 
         system = NodesSystem.objects.first()
-        self.assertEqual("UNDEFINED", system.nodes["node"].parameters["param"].macros["test"].value)
+        self.assertEqual(
+            "UNDEFINED",
+            system.nodes["node"].parameters["param"].macros["test"].value)
 
     def test_embedded_document_equality(self):
 
@@ -2730,5 +2734,26 @@ class InstanceTest(unittest.TestCase):
         self.assertEquals(p.id, None)
         p.id = "12345"  # in case it is not working: "OperationError: Shard Keys are immutable..." will be raised here
 
+    def test_null_field(self):
+        # 734
+        class User(Document):
+            name = StringField()
+            height = IntField(default=184, null=True)
+        User.objects.delete()
+        u = User(name='user')
+        u.save()
+        u_from_db = User.objects.get(name='user')
+        u_from_db.height = None
+        u_from_db.save()
+        self.assertEquals(u_from_db.height, None)
+
+        # 735
+        User.objects.delete()
+        u = User(name='user')
+        u.save()
+        User.objects(name='user').update_one(set__height=None, upsert=True)
+        u_from_db = User.objects.get(name='user')
+        self.assertEquals(u_from_db.height, None)
+
 if __name__ == '__main__':
     unittest.main()