Cannot use ignore_not_sysadmin validator

[mmeisel]

I'm trying to create an organization field that is only editable by sysadmins. I don't care if org admins can see it, but I don't want them to be able to change it (or clear it). Here's my attempt at such a field:

    {
      "field_name": "request_limit",
      "label": "Request Limit",
      "form_placeholder": "0",
      "validators": "ignore_not_sysadmin"
    }

But when I try to edit the organization as a non-sysadmin, I get this exception (I guess because the ignore_not_sysadmin validator deletes the key):

Traceback (most recent call last):
  File "/usr/lib/python3.8/site-packages/flask/app.py", line 1949, in full_dispatch_request
    rv = self.dispatch_request()
  File "/usr/lib/python3.8/site-packages/flask/app.py", line 1935, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/usr/lib/python3.8/site-packages/flask/views.py", line 89, in view
    return self.dispatch_request(*args, **kwargs)
  File "/usr/lib/python3.8/site-packages/flask/views.py", line 163, in dispatch_request
    return meth(*args, **kwargs)
  File "/srv/app/src/ckan/ckan/config/middleware/../../views/group.py", line 955, in post
    group = _action(u'group_update')(context, data_dict)
  File "/srv/app/src/ckan/ckan/logic/__init__.py", line 504, in wrapped
    result = _action(context, data_dict, **kw)
  File "/srv/app/src/ckan/ckan/logic/action/update.py", line 812, in organization_update
    return _group_or_org_update(context, data_dict, is_org=True)
  File "/srv/app/src/ckan/ckan/logic/action/update.py", line 684, in _group_or_org_update
    data, errors = lib_plugins.plugin_validate(
  File "/srv/app/src/ckan/ckan/lib/plugins.py", line 312, in plugin_validate
    result = plugin.validate(context, data_dict, schema, action)
  File "/srv/app/src/ckanext-scheming/ckanext/scheming/plugins.py", line 191, in validate
    return navl_validate(data_dict, schema, context)
  File "/srv/app/src/ckan/ckan/lib/navl/dictization_functions.py", line 285, in validate
    converted_data, errors = _validate(flattened, schema, validators_context)
  File "/srv/app/src/ckan/ckan/lib/navl/dictization_functions.py", line 335, in _validate
    convert(converter, key, converted_data, errors, context)
  File "/srv/app/src/ckan/ckan/lib/navl/dictization_functions.py", line 237, in convert
    converter(key, converted_data, errors, context)
  File "/srv/app/src/ckan/ckan/logic/converters.py", line 24, in convert_to_extras
    data[('extras', new_index, 'value')] = data[key]
KeyError: ('forecast_daily_locations_limit',)

If I add ignore_missing to the validators, no exception is raised, but editing the org as an org admin clears the field.

Any way to accomplish this?

Thanks!