Charging nothing to a client?

Charging nothing to a client?

I do pro bono work for a client. My accountant really wants me to keep track of how many hours I spend on this client's project (he's seen that sink other clients). Zoho, however, won't let me charge the client nothing. For now, I'm charging them a penny and never invoicing them, but that's really bugging me.

Can we allow for charging nothing for a certain task or client?