Using field variables does not work

I am using influxdb to store my data. In Grafana, I created variable from tag, that works fine. When I create variable from field, it stops working. Variable fk , shortcut for field key, can be amount or price. When I check generated query, it looks like this:

SELECT "/^amount$/" FROM "history"

I log to influxdb and test it, the result is empty. When I change it to:

SELECT "amount" FROM "history"

I can see some results. My questions is, why field variable does not work?