DjangoUnicodeDecodeError during json encoding of form errors

I am porting my site to django 1.9 and don't know how to resolve this issue correctly.

In my form I have usual EmailField from django forms. If validation fails, it shoud be message about it (I pass 'form_errors': dict(form.errors.items())} to context for manipulating).

But in that case django returns

DjangoUnicodeDecodeError: 'ascii' codec can't decode byte 0xd0 in position 0: ordinal not in range(128). You passed in ()

in django.core.validators there is a validator for it

@deconstructible
class EmailValidator(object):
    message = _('Enter a valid email address.')
    ...

If I change message to message = 'error' all works fine.

So, question: how can I fix this issue without editing django files?

UPDATE

the problem is in json responsing

in view process looks like that now:

from django.http import JsonResponse
...

if form.is_valid():
    ...
else:
    return JsonResponse({'form_errors': form.errors})

last traceback is:

  File "/path/views.py", line 331, in custom_form_post
    response = JsonResponse({'form_errors': form.errors})
  File "/path/.env/local/lib/python2.7/site-packages/django/http/response.py", line 505, in __init__
    data = json.dumps(data, cls=encoder, **json_dumps_params)
  File "/usr/lib/python2.7/json/__init__.py", line 250, in dumps
    sort_keys=sort_keys, **kw).encode(obj)
  File "/usr/lib/python2.7/json/encoder.py", line 207, in encode
    chunks = self.iterencode(o, _one_shot=True)
  File "/usr/lib/python2.7/json/encoder.py", line 270, in iterencode
    return _iterencode(o, 0)
  File "/path/.env/lib/python2.7/_abcoll.py", line 581, in __iter__
    v = self[i]
  File "/path/.env/local/lib/python2.7/site-packages/django/forms/utils.py", line 146, in __getitem__
    return list(error)[0]
  File "/path/.env/local/lib/python2.7/site-packages/django/core/exceptions.py", line 165, in __iter__
    yield force_text(message)
  File "/path/.env/local/lib/python2.7/site-packages/django/utils/encoding.py", line 88, in force_text
    raise DjangoUnicodeDecodeError(s, *e.args)
DjangoUnicodeDecodeError: 'ascii' codec can't decode byte 0xd0 in position 0: ordinal not in range(128). You passed in <django.utils.functional.__proxy__ object at 0x40a6a90c> (<class 'django.utils.functional.__proxy__'>)