email.policy.SMTP TypeError: 'Header' object is not subscriptable

Bug report

Code snippet is taken directly from documentation https://docs.python.org/3/library/email.header.html
The only change - policy changed to email.policy.SMTP.
Documentation (https://docs.python.org/3/library/email.policy.html#email.policy.SMTP) states that the only difference between email.policy.default and email.policy.SMTP - linesep used.

from email.message import Message
from email.header import Header
from email.policy import SMTP

msg = Message()
h = Header('p\xf6stal', 'iso-8859-1')
msg['Subject'] = h
print(msg.as_string())

msg = Message(policy=SMTP)
h = Header('p\xf6stal', 'iso-8859-1')
msg['Subject'] = h
print(msg.as_string())

Output is:

Subject: =?iso-8859-1?q?p=F6stal?=


Traceback (most recent call last):
  File "C:\Users\barsk\AppData\Roaming\JetBrains\PyCharm2022.3\scratches\scratch_3.py", line 12, in <module>
    msg['Subject'] = h
  File "C:\Program Files (x86)\Python37-32\lib\email\message.py", line 409, in __setitem__
    self._headers.append(self.policy.header_store_parse(name, val))
  File "C:\Program Files (x86)\Python37-32\lib\email\policy.py", line 148, in header_store_parse
    return (name, self.header_factory(name, value))
  File "C:\Program Files (x86)\Python37-32\lib\email\headerregistry.py", line 589, in __call__
    return self[name](name, value)
  File "C:\Program Files (x86)\Python37-32\lib\email\headerregistry.py", line 197, in __new__
    cls.parse(value, kwds)
  File "C:\Program Files (x86)\Python37-32\lib\email\headerregistry.py", line 272, in parse
    kwds['parse_tree'] = cls.value_parser(value)
  File "C:\Program Files (x86)\Python37-32\lib\email\_header_value_parser.py", line 1105, in get_unstructured
    if value[0] in WSP:
TypeError: 'Header' object is not subscriptable

Your environment

Reproduced on windows on

• Python 3.11 (64-bit)
• Python 3.10 (64-bit)
• Python 3.7 (32-bit)

Start by running the provided reproducer with email.message.Message, email.policy.SMTP, and email.header.Header. Trace the failure through email/message.py, email/policy.py, email/headerregistry.py, and email/_header_value_parser.py, then compare it with the default-policy behavior. Done means the SMTP-policy example no longer raises the reported TypeError and preserves the documented output behavior.