makemigrations failed when CharField has validators=[validate_slug]

[mrgaolei]

Version 1.9rc1
When CharField have a validators=[validate_slug], it can not be makemigrations.

it prompt:

Traceback (most recent call last):
  File "./manage.py", line 10, in <module>
    execute_from_command_line(sys.argv)
  File "/Users/mrgaolei/Desktop/proton/venv/lib/python2.7/site-packages/django/core/management/__init__.py", line 350, in execute_from_command_line
    utility.execute()
  File "/Users/mrgaolei/Desktop/proton/venv/lib/python2.7/site-packages/django/core/management/__init__.py", line 342, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/Users/mrgaolei/Desktop/proton/venv/lib/python2.7/site-packages/django/core/management/base.py", line 348, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/Users/mrgaolei/Desktop/proton/venv/lib/python2.7/site-packages/django/core/management/base.py", line 399, in execute
    output = self.handle(*args, **options)
  File "/Users/mrgaolei/Desktop/proton/venv/lib/python2.7/site-packages/django/core/management/commands/makemigrations.py", line 150, in handle
    self.write_migration_files(changes)
  File "/Users/mrgaolei/Desktop/proton/venv/lib/python2.7/site-packages/django/core/management/commands/makemigrations.py", line 178, in write_migration_files
    migration_string = writer.as_string()
  File "/Users/mrgaolei/Desktop/proton/venv/lib/python2.7/site-packages/django/db/migrations/writer.py", line 167, in as_string
    operation_string, operation_imports = OperationWriter(operation).serialize()
  File "/Users/mrgaolei/Desktop/proton/venv/lib/python2.7/site-packages/django/db/migrations/writer.py", line 124, in serialize
    _write(arg_name, arg_value)
  File "/Users/mrgaolei/Desktop/proton/venv/lib/python2.7/site-packages/django/db/migrations/writer.py", line 76, in _write
    arg_string, arg_imports = MigrationWriter.serialize(item)
  File "/Users/mrgaolei/Desktop/proton/venv/lib/python2.7/site-packages/django/db/migrations/writer.py", line 353, in serialize
    item_string, item_imports = cls.serialize(item)
  File "/Users/mrgaolei/Desktop/proton/venv/lib/python2.7/site-packages/django/db/migrations/writer.py", line 429, in serialize
    return cls.serialize_deconstructed(path, args, kwargs)
  File "/Users/mrgaolei/Desktop/proton/venv/lib/python2.7/site-packages/django/db/migrations/writer.py", line 318, in serialize_deconstructed
    arg_string, arg_imports = cls.serialize(arg)
  File "/Users/mrgaolei/Desktop/proton/venv/lib/python2.7/site-packages/django/db/migrations/writer.py", line 353, in serialize
    item_string, item_imports = cls.serialize(item)
  File "/Users/mrgaolei/Desktop/proton/venv/lib/python2.7/site-packages/django/db/migrations/writer.py", line 473, in serialize
    return cls.serialize_deconstructed(*value.deconstruct())
  File "/Users/mrgaolei/Desktop/proton/venv/lib/python2.7/site-packages/django/db/migrations/writer.py", line 314, in serialize_deconstructed
    arg_string, arg_imports = cls.serialize(arg)
  File "/Users/mrgaolei/Desktop/proton/venv/lib/python2.7/site-packages/django/db/migrations/writer.py", line 512, in serialize
    for item in value:
  File "/Users/mrgaolei/Desktop/proton/venv/lib/python2.7/site-packages/django/utils/functional.py", line 205, in inner
    return func(self._wrapped, *args)
TypeError: '_sre.SRE_Pattern' object is not iterable

[Baptiste Mispelon]

I can reproduce the issue indeed (couldn't figure out how to make a testcase for it though).

Bisecting the issue points to 2bb1027d6bcdad59624a9d08701e0d2e4a9c0ba7.

Thanks.