TypeError: __str__ returned non-string (type bytes)

[Mahonghui]

Traceback (most recent call last):
File "/Users/mahonghui/Desktop/daily_fresh/df/lib/python3.7/site-packages/django/core/handlers/exception.py", line 34, in inner
response = get_response(request)
File "/Users/mahonghui/Desktop/daily_fresh/df/lib/python3.7/site-packages/django/core/handlers/base.py", line 126, in _get_response
response = self.process_exception_by_middleware(e, request)
File "/Users/mahonghui/Desktop/daily_fresh/df/lib/python3.7/site-packages/django/core/handlers/base.py", line 124, in _get_response
response = wrapped_callback(request, *callback_args, **callback_kwargs)
File "/Users/mahonghui/Desktop/daily_fresh/df/lib/python3.7/site-packages/django/contrib/admin/options.py", line 604, in wrapper
return self.admin_site.admin_view(view)(*args, **kwargs)
File "/Users/mahonghui/Desktop/daily_fresh/df/lib/python3.7/site-packages/django/utils/decorators.py", line 142, in _wrapped_view
response = view_func(request, *args, **kwargs)
File "/Users/mahonghui/Desktop/daily_fresh/df/lib/python3.7/site-packages/django/views/decorators/cache.py", line 44, in _wrapped_view_func
response = view_func(request, *args, **kwargs)
File "/Users/mahonghui/Desktop/daily_fresh/df/lib/python3.7/site-packages/django/contrib/admin/sites.py", line 223, in inner
return view(request, *args, **kwargs)
File "/Users/mahonghui/Desktop/daily_fresh/df/lib/python3.7/site-packages/django/contrib/admin/options.py", line 1636, in add_view
return self.changeform_view(request, None, form_url, extra_context)
File "/Users/mahonghui/Desktop/daily_fresh/df/lib/python3.7/site-packages/django/utils/decorators.py", line 45, in _wrapper
return bound_method(*args, **kwargs)
File "/Users/mahonghui/Desktop/daily_fresh/df/lib/python3.7/site-packages/django/utils/decorators.py", line 142, in _wrapped_view
response = view_func(request, *args, **kwargs)
File "/Users/mahonghui/Desktop/daily_fresh/df/lib/python3.7/site-packages/django/contrib/admin/options.py", line 1525, in changeform_view
return self._changeform_view(request, object_id, form_url, extra_context)
File "/Users/mahonghui/Desktop/daily_fresh/df/lib/python3.7/site-packages/django/contrib/admin/options.py", line 1564, in _changeform_view
self.save_model(request, new_object, form, not add)
File "/Users/mahonghui/Desktop/daily_fresh/df/lib/python3.7/site-packages/django/contrib/admin/options.py", line 1091, in save_model
obj.save()
File "/Users/mahonghui/Desktop/daily_fresh/df/lib/python3.7/site-packages/django/db/models/base.py", line 718, in save
force_update=force_update, update_fields=update_fields)
File "/Users/mahonghui/Desktop/daily_fresh/df/lib/python3.7/site-packages/django/db/models/base.py", line 748, in save_base
updated = self._save_table(raw, cls, force_insert, force_update, using, update_fields)
File "/Users/mahonghui/Desktop/daily_fresh/df/lib/python3.7/site-packages/django/db/models/base.py", line 831, in _save_table
result = self._do_insert(cls._base_manager, using, fields, update_pk, raw)
File "/Users/mahonghui/Desktop/daily_fresh/df/lib/python3.7/site-packages/django/db/models/base.py", line 869, in _do_insert
using=using, raw=raw)
File "/Users/mahonghui/Desktop/daily_fresh/df/lib/python3.7/site-packages/django/db/models/manager.py", line 82, in manager_method
return getattr(self.get_queryset(), name)(*args, **kwargs)
File "/Users/mahonghui/Desktop/daily_fresh/df/lib/python3.7/site-packages/django/db/models/query.py", line 1136, in _insert
return query.get_compiler(using=using).execute_sql(return_id)
File "/Users/mahonghui/Desktop/daily_fresh/df/lib/python3.7/site-packages/django/db/models/sql/compiler.py", line 1288, in execute_sql
for sql, params in self.as_sql():
File "/Users/mahonghui/Desktop/daily_fresh/df/lib/python3.7/site-packages/django/db/models/sql/compiler.py", line 1241, in as_sql
for obj in self.query.objs
File "/Users/mahonghui/Desktop/daily_fresh/df/lib/python3.7/site-packages/django/db/models/sql/compiler.py", line 1241, in
for obj in self.query.objs
File "/Users/mahonghui/Desktop/daily_fresh/df/lib/python3.7/site-packages/django/db/models/sql/compiler.py", line 1240, in
[self.prepare_value(field, self.pre_save_val(field, obj)) for field in fields]
File "/Users/mahonghui/Desktop/daily_fresh/df/lib/python3.7/site-packages/django/db/models/sql/compiler.py", line 1182, in prepare_value
value = field.get_db_prep_save(value, connection=self.connection)
File "/Users/mahonghui/Desktop/daily_fresh/df/lib/python3.7/site-packages/django/db/models/fields/init.py", line 790, in get_db_prep_save
return self.get_db_prep_value(value, connection=connection, prepared=False)
File "/Users/mahonghui/Desktop/daily_fresh/df/lib/python3.7/site-packages/django/db/models/fields/init.py", line 785, in get_db_prep_value
value = self.get_prep_value(value)
File "/Users/mahonghui/Desktop/daily_fresh/df/lib/python3.7/site-packages/django/db/models/fields/files.py", line 282, in get_prep_value
return str(value)
TypeError: str returned non-string (type bytes)