LEFT OUTER JOIN on the same bucket

SQL++
,
1

Hi, I have created the following index:
CREATE PRIMARY INDEXfacts-primary-indexONfactsUSING GSI;

I have the following query that performs LEFT OUTER JOIN:
SELECT f1.creator FROM facts as f1 LEFT OUTER JOIN facts as f2 ON KEYS f2.creator WHERE f1.factType='HEARTBEAT_FACT';

For some reason I get the following error:
{
"code": 3000,
"msg": "Ambiguous reference to field f2.",
"query_from_user": "SELECT f1.creator FROM facts as f1\nLEFT OUTER JOIN facts as f2 ON KEYS f2.creator\nWHERE f1.factType='HEARTBEAT_FACT';"
}

What do I missing?
Did I created all the necessary indices?
Do I need to add something to the query?

2

SELECT f1.creator FROM facts as f1 LEFT OUTER JOIN facts as f2 ON KEYS f1.creator WHERE f1.factType=‘HEARTBEAT_FACT’;

ON KEYS expression requires from left side expression