Getting Error While Saving the Doctype

Traceback (most recent call last):
File “apps/frappe/frappe/app.py”, line 69, in application
response = frappe.api.handle()
File “apps/frappe/frappe/api.py”, line 55, in handle
return frappe.handler.handle()
File “apps/frappe/frappe/handler.py”, line 38, in handle
data = execute_cmd(cmd)
File “apps/frappe/frappe/handler.py”, line 76, in execute_cmd
return frappe.call(method, **frappe.form_dict)
File “apps/frappe/frappe/init.py”, line 1448, in call
return fn(*args, **newargs)
File “apps/frappe/frappe/desk/form/save.py”, line 16, in savedocs
doc = frappe.get_doc(json.loads(doc))
File “apps/frappe/frappe/init.py”, line 1068, in get_doc
doc = frappe.model.document.get_doc(*args, **kwargs)
File “apps/frappe/frappe/model/document.py”, line 77, in get_doc
controller = get_controller(doctype)
File “apps/frappe/frappe/model/base_document.py”, line 78, in get_controller
return _get_controller()
File “apps/frappe/frappe/model/base_document.py”, line 74, in _get_controller
raise ImportError(doctype)
ImportError: Assignment

any help ??