decrementing Decimal raises InvalidDocument exception type

[ghost]

I have models like

import mongoengine as me

class Orders(me.Document):
         order_id = me.SequenceField(primary_key=True)
         total_due_amount = me.DecimalField(required=True)
         # other  irrevelant fields 

I have views like

def update_total_due_amount(request):
      # some irrelevant pre processing
      total_amount_paid = 10    # after some preprocessing assume total_amount_paid is 10
      order_obj_update = Orders.objects(order_id=order_id).update_one(dec__total_due_amount=total_amount_paid) # this raises an exception 

The actual exception is Cannot encode object: Decimal('-10.00')
I have worked around by

Orders.objects(order_id=order_id).update_one(dec__total_due_amount=int(total_amount_paid)) # observe the int()

Is this how it should have been done ? Or am I missing something ?