Hey all,
I'm running this test:
```
import requests
import pandas as pd
url = 'https://api.patentsview.org/patents/query'
r = requests.post(url, json={ "q": {"_and": [{'_text_any': {"patent_title": "renal"}}, {'_text_any': {"patent_title": "cell"}}, {'_contains': {'cpc_subsection_id': 'A61'}}, {'_gte': {'patent_date': '2015-01-01'}}] }, "f": ["patent_title", "patent_date", "patent_country"], "o": {"page": 1, "per_page": 100}})
print(r.json())
la = pd.json_normalize(r.json()['patents'])
```
and I get the error: `{'status': 'error', 'payload': {'error': 'Invalid field specified: patent_country ', 'code': 'PINV8'}}`
Am I using this endpoint wrong? or is it not functioning properly?