500 error happened when PATCH documents with procedure

[2020-09-29T14:45:34.523Z] | prozorro_sale.registry.errors | ERROR - Unknown error caught in API - 'str' object has no attribute 'items' | X-Request-ID: fe24c561-849c-4dac-8bee-990d616f924f
Traceback (most recent call last):
  File "/registry/prozorro_sale/registry/errors.py", line 53, in request_errors_middleware
    return await handler(request)
  File "/registry/prozorro_sale/registry/auth.py", line 58, in context_middleware
    return await handler(request)
  File "/registry/prozorro_sale/registry/auth.py", line 65, in access_middleware
    return await check_access(handler)(request)
  File "/registry/prozorro_sale/registry/auth.py", line 74, in handler
    return await func(request, *args, **kwargs)
  File "/usr/local/lib/python3.8/site-packages/prozorro_sale/tools/__init__.py", line 81, in retry_on_concurrency_error_middleware
    return await handler(request)
  File "/usr/local/lib/python3.8/site-packages/prozorro_sale/tools/__init__.py", line 67, in handler
    return await func(request, *args, **kwargs)
  File "/usr/local/lib/python3.8/site-packages/prozorro_sale/tools/__init__.py", line 45, in request_unpack_params
    return await handler(request, **request.match_info)
  File "/registry/prozorro_sale/registry/api.py", line 52, in update_registry_object
    registry_obj.update(data, request._context)
  File "/registry/prozorro_sale/registry/models/base.py", line 217, in update
    self.import_data(raw_data=data, role='owner')
  File "/usr/local/lib/python3.8/site-packages/schematics/models.py", line 171, in import_data
    return convert(self.__class__, self, raw_data, role=role, **kwargs)
  File "/usr/local/lib/python3.8/site-packages/schematics/transforms.py", line 461, in convert
    return import_loop(cls, mutable, raw_data, validation_converter, **kwargs)
  File "/usr/local/lib/python3.8/site-packages/schematics/transforms.py", line 200, in import_loop
    setattr(mutable, field_name, value)
  File "/usr/local/lib/python3.8/site-packages/schematics/models.py", line 34, in _setter
    getattr(self, pre_set_callback_name)(value)
  File "/registry/prozorro_sale/registry/models/utils.py", line 14, in _on_documents_set
    if self.documents and doc.id in self.documents:
  File "/registry/prozorro_sale/registry/models/base.py", line 191, in __eq__
    for field, value in other.items():
AttributeError: 'str' object has no attribute 'items'
[2020-09-29T14:45:34.524Z] | aiohttp.access | INFO - "176.36.60.12 PATCH /api/registry/5f733037d18c5b6461b2e4cb?acc_token=%23censored%23 HTTP/1.1 500 264 - GuzzleHttp/6.5.3 curl/7.68.0 PHP/7.4.9 0.005786 " | X-Request-ID: fe24c561-849c-4dac-8bee-990d616f924f