diff --git a/mongoengine/fields.py b/mongoengine/fields.py
index 26999204..e1b43664 100644
--- a/mongoengine/fields.py
+++ b/mongoengine/fields.py
@@ -604,6 +604,11 @@ class ReferenceField(BaseField):
     def validate(self, value):
         assert isinstance(value, (self.document_type, pymongo.dbref.DBRef))
 
+        if isinstance(value, Document) and value.id is None:
+            raise ValidationError('You can only reference documents once '
+                                  'they have been saved to the database')
+
+
     def lookup_member(self, member_name):
         return self.document_type._fields.get(member_name)
 
@@ -628,6 +633,15 @@ class GenericReferenceField(BaseField):
 
         return super(GenericReferenceField, self).__get__(instance, owner)
 
+    def validate(self, value):
+        if not isinstance(value, (Document, pymongo.dbref.DBRef)):
+            raise ValidationError('GenericReferences can only contain documents')
+
+        # We need the id from the saved object to create the DBRef
+        if isinstance(value, Document) and value.id is None:
+            raise ValidationError('You can only reference documents once '
+                                  'they have been saved to the database')
+
     def dereference(self, value):
         doc_cls = get_document(value['_cls'])
         reference = value['_ref']
diff --git a/tests/fields.py b/tests/fields.py
index c13f9e34..22049309 100644
--- a/tests/fields.py
+++ b/tests/fields.py
@@ -377,6 +377,7 @@ class FieldTest(unittest.TestCase):
             comments = ListField(EmbeddedDocumentField(Comment))
             tags = ListField(StringField())
             authors = ListField(ReferenceField(User))
+            generic = ListField(GenericReferenceField())
 
         post = BlogPost(content='Went for a walk today...')
         post.validate()
@@ -404,8 +405,28 @@ class FieldTest(unittest.TestCase):
         self.assertRaises(ValidationError, post.validate)
 
         post.authors = [User()]
+        self.assertRaises(ValidationError, post.validate)
+
+        user = User()
+        user.save()
+        post.authors = [user]
         post.validate()
 
+        post.generic = [1, 2]
+        self.assertRaises(ValidationError, post.validate)
+
+        post.generic = [User(), Comment()]
+        self.assertRaises(ValidationError, post.validate)
+
+        post.generic = [Comment()]
+        self.assertRaises(ValidationError, post.validate)
+
+        post.generic = [user]
+        post.validate()
+
+        User.drop_collection()
+        BlogPost.drop_collection()
+
     def test_sorted_list_sorting(self):
         """Ensure that a sorted list field properly sorts values.
         """