Can't save new key in DynamicField

Hi, i get some trouble with using DynamicField with dict as default value, there example of my code

class AuthorBooks(Document):
    author = StringField()
    comments = DynamicField(default=dict)

a = AuthorBooks()
a.author = "Andrew"
a.comments['booklist'] = ['Moby Dick']
a.save()
a.save()
a.comments['description'] = 'whale'
a.save()

In mongo we got only

{
    "_id" : ObjectId("54eae2822796251b080e3219"),
    "author" : "Andrew",
    "comments" : {
        "booklist" : [
            "Moby Dick"
        ]
    }
}

if change object id by copy.copy() save correclty

class AuthorBooks(Document):
    author = StringField()
    comments = DynamicField(default=dict)


a = AuthorBooks()
a.author = "Andrew"
a.comments['booklist'] = ['Moby Dick']
a.save()
comments = copy.copy(a.comments)
comments['description'] = 'whale'
a.comments = comments
a.save()

Mongo

{
    "_id" : ObjectId("54eae2822796251b080e3219"),
    "author" : "Andrew",
    "comments" : {
        "booklist" : [
            "Moby Dick"
        ],
        "description" : "whale"
    }
}

Start with DynamicField and the change-tracking path, then reproduce the reported sequence of mutating comments, saving, mutating it again, and saving once more. Done means the second in-place mutation, including the description key, is persisted without copying and reassigning the dictionary.