Firebase Functions - Billing account not configured

I just implemented the new Firebase Functions , but there something in the log that bothers me:

Billing account not configured. External network is not accessible and quotas are severily limited. Configure billing account to remove these restrictions

I actually managed to access the function from the external network, so I'm wondering do I really have to give my billing account? There's nothing about this in the firebase documentation.

I'm using the Spark subscription plan for personal test purposes and I don't want to add billing info if I'm not going to pay for anything yet.

via Hristo Stoyanov