E-sign : employee cannot sign at the end of the doc

Error:
Odoo Server Error

Traceback (most recent call last):
  File "/home/odoo/src/odoo/13.0/odoo/models.py", line 4978, in ensure_one
    _id, = self._ids
ValueError: too many values to unpack (expected 1)

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/odoo/src/odoo/13.0/odoo/http.py", line 619, in _handle_exception
    return super(JsonRequest, self)._handle_exception(exception)
  File "/home/odoo/src/odoo/13.0/odoo/http.py", line 309, in _handle_exception
    raise pycompat.reraise(type(exception), exception, sys.exc_info()[2])
  File "/home/odoo/src/odoo/13.0/odoo/tools/pycompat.py", line 14, in reraise
    raise value
  File "/home/odoo/src/odoo/13.0/odoo/http.py", line 664, in dispatch
    result = self._call_function(**self.params)
  File "/home/odoo/src/odoo/13.0/odoo/http.py", line 345, in _call_function
    return checked_call(self.db, *args, **kwargs)
  File "/home/odoo/src/odoo/13.0/odoo/service/model.py", line 93, in wrapper
    return f(dbname, *args, **kwargs)
  File "/home/odoo/src/odoo/13.0/odoo/http.py", line 338, in checked_call
    result = self.endpoint(*a, **kw)
  File "/home/odoo/src/odoo/13.0/odoo/http.py", line 910, in __call__
    return self.method(*args, **kw)
  File "/home/odoo/src/odoo/13.0/odoo/http.py", line 510, in response_wrap
    response = f(*args, **kw)
  File "/home/odoo/src/enterprise/13.0/sign/controllers/main.py", line 225, in sign_get_user_signature
    return sign_request_user.sign_signature
  File "/home/odoo/src/odoo/13.0/odoo/fields.py", line 984, in __get__
    record.ensure_one()
  File "/home/odoo/src/odoo/13.0/odoo/models.py", line 4981, in ensure_one
    raise ValueError("Expected singleton: %s" % self)
ValueError: Expected singleton: res.users(946563, 949724)