diff --git a/docs/changelog.rst b/docs/changelog.rst
index 809650f3..e3f3ecd1 100644
--- a/docs/changelog.rst
+++ b/docs/changelog.rst
@@ -5,6 +5,7 @@ Changelog
 
 Changes in 0.9.X - DEV
 ======================
+- 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
 - Querying by a field defined in a subclass raises InvalidQueryError #744
diff --git a/mongoengine/base/document.py b/mongoengine/base/document.py
index bf5bdf79..8995a304 100644
--- a/mongoengine/base/document.py
+++ b/mongoengine/base/document.py
@@ -386,9 +386,9 @@ class BaseDocument(object):
         return json_util.dumps(self.to_mongo(use_db_field),  *args, **kwargs)
 
     @classmethod
-    def from_json(cls, json_data):
+    def from_json(cls, json_data, created=False):
         """Converts json data to an unsaved document instance"""
-        return cls._from_son(json_util.loads(json_data))
+        return cls._from_son(json_util.loads(json_data), created=created)
 
     def __expand_dynamic_values(self, name, value):
         """expand any dynamic values to their correct types / values"""
@@ -617,7 +617,7 @@ class BaseDocument(object):
         return cls._meta.get('collection', None)
 
     @classmethod
-    def _from_son(cls, son, _auto_dereference=True, only_fields=[]):
+    def _from_son(cls, son, _auto_dereference=True, only_fields=[], created=False):
         """Create an instance of a Document (subclass) from a PyMongo SON.
         """
 
@@ -667,7 +667,7 @@ class BaseDocument(object):
         if cls.STRICT:
             data = dict((k, v)
                         for k, v in data.iteritems() if k in cls._fields)
-        obj = cls(__auto_convert=False, _created=False, __only_fields=only_fields, **data)
+        obj = cls(__auto_convert=False, _created=created, __only_fields=only_fields, **data)
         obj._changed_fields = changed_fields
         if not _auto_dereference:
             obj._fields = fields
diff --git a/tests/document/instance.py b/tests/document/instance.py
index a22f3fbf..b7733548 100644
--- a/tests/document/instance.py
+++ b/tests/document/instance.py
@@ -2719,5 +2719,16 @@ class InstanceTest(unittest.TestCase):
         self.assertEquals(p4.height, 189)
         self.assertEquals(Person.objects(height=189).count(), 1)
 
+    def test_from_son(self):
+        # 771
+        class MyPerson(self.Person):
+            meta = dict(shard_key=["id"])
+        p = MyPerson.from_json('{"name": "name", "age": 27}', created=True)
+        self.assertEquals(p.id, None)
+        p.id = "12345"  # in case it is not working: "OperationError: Shard Keys are immutable..." will be raised here
+        p = MyPerson._from_son({"name": "name", "age": 27}, created=True)
+        self.assertEquals(p.id, None)
+        p.id = "12345"  # in case it is not working: "OperationError: Shard Keys are immutable..." will be raised here
+
 if __name__ == '__main__':
     unittest.main()