The 'fields' option is ignored in search.nodes

I'm not sure if this is a json-rpc or services issue so apologies before I begin.

I'm using the following Python to access search.nodes:

		data = {
			'hash': self._hash,
			'domain_name': self._domain,
			'domain_time_stamp': self._timestamp,
			'nonce': self._nonce,
			'sessid': self._session_id,
			'search_keys': keywords,
			'fields': fields,
		}
		if simple == True:
			data['simple'] = 'true'

If I set the simple field then I get the basic fields (link, type, title, user, date, snippet).:

data['simple'] = 'true'

If I don't send 'simple' but I do send 'fields' as shown then I receive ALL fields for each node:

data['fields'] = ['nid','title']

I would expect to only receive the nid and title for each node found.

Thanks