diff --git a/AUTHORS b/AUTHORS
index 28b57bb4..bcf6129e 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -80,3 +80,4 @@ that much better:
  * Stephen Young
  * tkloc
  * aid
+ * yamaneko1212
diff --git a/docs/changelog.rst b/docs/changelog.rst
index d510531f..602294a1 100644
--- a/docs/changelog.rst
+++ b/docs/changelog.rst
@@ -5,6 +5,7 @@ Changelog
 Changes in dev
 ==============
 
+- Fixed Handle None values for non-required fields.
 - Removed Document._get_subclasses() - no longer required
 - Fixed bug requiring subclasses when not actually needed
 - Fixed deletion of dynamic data
diff --git a/mongoengine/base.py b/mongoengine/base.py
index 402f191e..ad2ebca7 100644
--- a/mongoengine/base.py
+++ b/mongoengine/base.py
@@ -751,6 +751,13 @@ class BaseDocument(object):
                 self._data[name] = value
                 if hasattr(self, '_changed_fields'):
                     self._mark_as_changed(name)
+
+        # Handle None values for required fields
+        if value is None and name in getattr(self, '_fields', {}):
+            self._data[name] = value
+            if hasattr(self, '_changed_fields'):
+                self._mark_as_changed(name)
+            return
         super(BaseDocument, self).__setattr__(name, value)
 
     def __expand_dynamic_values(self, name, value):
diff --git a/mongoengine/queryset.py b/mongoengine/queryset.py
index 6025dd99..308bdf7f 100644
--- a/mongoengine/queryset.py
+++ b/mongoengine/queryset.py
@@ -694,8 +694,7 @@ class QuerySet(object):
                 elif op in ('in', 'nin', 'all', 'near'):
                     # 'in', 'nin' and 'all' require a list of values
                     value = [field.prepare_query_value(op, v) for v in value]
-                
-                
+
             # if op and op not in match_operators:
             if op:
                 if op in geo_operators:
@@ -1301,7 +1300,8 @@ class QuerySet(object):
                 field = cleaned_fields[-1]
 
                 if op in (None, 'set', 'push', 'pull', 'addToSet'):
-                    value = field.prepare_query_value(op, value)
+                    if field.required or value is not None:
+                        value = field.prepare_query_value(op, value)
                 elif op in ('pushAll', 'pullAll'):
                     value = [field.prepare_query_value(op, v) for v in value]
 
diff --git a/tests/fields.py b/tests/fields.py
index 0bc15a3c..37972fd0 100644
--- a/tests/fields.py
+++ b/tests/fields.py
@@ -46,6 +46,81 @@ class FieldTest(unittest.TestCase):
         person = Person(age=30)
         self.assertRaises(ValidationError, person.validate)
 
+    def test_not_required_handles_none_in_update(self):
+        """Ensure that every fields should accept None if required is False.
+        """
+
+        class HandleNoneFields(Document):
+            str_fld = StringField()
+            int_fld = IntField()
+            flt_fld = FloatField()
+            comp_dt_fld = ComplexDateTimeField()
+
+        HandleNoneFields.drop_collection()
+
+        doc = HandleNoneFields()
+        doc.str_fld = u'spam ham egg'
+        doc.int_fld = 42
+        doc.flt_fld = 4.2
+        doc.com_dt_fld = datetime.datetime.utcnow()
+        doc.save()
+
+        res = HandleNoneFields.objects(id=doc.id).update(
+            set__str_fld=None,
+            set__int_fld=None,
+            set__flt_fld=None,
+            set__comp_dt_fld=None,
+        )
+        self.assertEqual(res, 1)
+
+        # Retrive data from db and verify it.
+        ret = HandleNoneFields.objects.all()[0]
+
+        self.assertEqual(ret.str_fld, None)
+        self.assertEqual(ret.int_fld, None)
+        self.assertEqual(ret.flt_fld, None)
+
+        # Return current time if retrived value is None.
+        self.assertTrue(isinstance(ret.comp_dt_fld, datetime.datetime))
+
+    def test_not_required_handles_none_from_database(self):
+        """Ensure that every fields can handle null values from the database.
+        """
+
+        class HandleNoneFields(Document):
+            str_fld = StringField(required=True)
+            int_fld = IntField(required=True)
+            flt_fld = FloatField(required=True)
+            comp_dt_fld = ComplexDateTimeField(required=True)
+
+        HandleNoneFields.drop_collection()
+
+        doc = HandleNoneFields()
+        doc.str_fld = u'spam ham egg'
+        doc.int_fld = 42
+        doc.flt_fld = 4.2
+        doc.com_dt_fld = datetime.datetime.utcnow()
+        doc.save()
+
+        collection = self.db[HandleNoneFields._get_collection_name()]
+        obj = collection.update({"_id": doc.id}, {"$unset": {
+            "str_fld": 1,
+            "int_fld": 1,
+            "flt_fld": 1,
+            "comp_dt_fld": 1}
+        })
+
+        # Retrive data from db and verify it.
+        ret = HandleNoneFields.objects.all()[0]
+
+        self.assertEqual(ret.str_fld, None)
+        self.assertEqual(ret.int_fld, None)
+        self.assertEqual(ret.flt_fld, None)
+        # Return current time if retrived value is None.
+        self.assert_(isinstance(ret.comp_dt_fld, datetime.datetime))
+
+        self.assertRaises(ValidationError, ret.validate)
+
     def test_object_id_validation(self):
         """Ensure that invalid values cannot be assigned to string fields.
         """