Middleware fatal error when saving Samba share after updating to 12

I have a samba share with pre-existing ACLs. I had to edit the share, and it of course refused to save because it (correctly) detected ACLs in use. I checked the "Enable ACLs" checkbox, clicked Save again, and got an unspecified(!) error and traceback.

Error: Traceback (most recent call last):
  File "/usr/local/lib/python3.8/site-packages/middlewared/main.py", line 137, in call_method
    result = await self.middleware._call(message['method'], serviceobj, methodobj, params, app=self,
  File "/usr/local/lib/python3.8/site-packages/middlewared/main.py", line 1187, in _call
    return await methodobj(*prepared_call.args)
  File "/usr/local/lib/python3.8/site-packages/middlewared/service.py", line 457, in update
    rv = await self.middleware._call(
  File "/usr/local/lib/python3.8/site-packages/middlewared/main.py", line 1187, in _call
    return await methodobj(*prepared_call.args)
  File "/usr/local/lib/python3.8/site-packages/middlewared/schema.py", line 973, in nf
    return await f(*args, **kwargs)
  File "/usr/local/lib/python3.8/site-packages/middlewared/plugins/smb.py", line 743, in do_update
    diff = await self.middleware.call(
  File "/usr/local/lib/python3.8/site-packages/middlewared/main.py", line 1229, in call
    return await self._call(
  File "/usr/local/lib/python3.8/site-packages/middlewared/main.py", line 1187, in _call
    return await methodobj(*prepared_call.args)
  File "/usr/local/lib/python3.8/site-packages/middlewared/plugins/smb_/registry.py", line 172, in diff_middleware_and_registry
    reg_conf = await self.reg_showshare(share if not data['home'] else 'homes')
  File "/usr/local/lib/python3.8/site-packages/middlewared/plugins/smb_/registry.py", line 78, in reg_showshare
    net = await self.netconf(action='showshare', share=share)
  File "/usr/local/lib/python3.8/site-packages/middlewared/plugins/smb_/registry.py", line 46, in netconf
    raise CallError(
middlewared.service_exception.CallError: [EFAULT] net conf showshare [User_files_(Main)] failed with error: