I have a `Page` model as follow and the `page_id` field isn't `unique` but I can't create an instance from `Page` and this error rises 

`OperationError: Could not save document (insertDocument :: caused by ::11000 E11000 duplicate key error index: shopify.page.$pageId_1 dup key: { : null })`
I have no idea that why the duplicate key error rises when the `page_id` field isn't `unique`

The `Site` model : 

```
class Page(Document):

    # page identity
    page_id = StringField()
    store = ReferenceField('Store')
    is_product = BooleanField(default = False)
    is_homepage = BooleanField(default = False)
    product = ReferenceField('Product')
    requests = ListField(EmbeddedDocumentField('Request'))

    # page stat
    visitors = IntField(default = 0)
    views = IntField(default = 0)
    past_days = ListField(EmbeddedDocumentField('DayStat'))

```