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AttributeError: 'dict' object has no attribute 'modified'

Hi All,
i am try to update document in server side . but i am getting error as below
AttributeError: ‘dict’ object has no attribute ‘modified’
i see this error is coming while updating child table values. here is my code

risk_assesment_doc = frappe.get_doc('Risk Assesment', 'RA-2021-00028')
risk_assesment_doc.assesment_detail = assesment_detail //value for child table
risk_assesment_doc.total = total_score
risk_assesment_doc.save()
frappe.db.commit()

inserting new document is working fine, but updating is not working