MongoEngine: Embeddeddocument fields do not take default value as none?

I've set the default values for fields in embedded document but when I try to post data it doesn't accept None or Blank values.

Here is what my code looks like-

models.py

class MetaData(EmbeddedDocument):
    adcode = StringField(max_length=50, default="", blank=True, Null=True)
    additional_html_below_header = StringField(max_length=50, default="")
    adhoc_plus_disable_pacing = BooleanField(default=False)
    adhoc_plus_has_priority = BooleanField(default=False)
    adhoc_server = StringField(max_length=50, default="")
class LandingPage(Document):
    type = StringField(max_length=50, default="")
    meta_clean_URL_tag = StringField(max_length=50, default="")
    meta_name =  StringField(max_length=50, default="")
    created_time = DateTimeField(default=datetime.datetime.now)
    new = BooleanField(default=False)
    meta_data = EmbeddedDocumentField(MetaData)

serializers.py

class MetaDataSerializer(serializers.EmbeddedDocumentSerializer):
    class Meta:
        model = MetaData

class LandingPageSerializer(serializers.DocumentSerializer):
    meta_data = MetaDataSerializer()
    class Meta:
        model = LandingPage

Is there anything wrong I'm doing here?

Start by reviewing the field declarations in models.py and the EmbeddedDocumentSerializer and DocumentSerializer definitions in serializers.py. Reproduce the reported handling of None and blank values, then establish whether the model defaults or serializer behavior is responsible; the issue is complete when the expected embedded-document behavior is defined and verified.