OPC 1.1.0 (yes, I know it's old but I didn't see any reference in version changes that would have addressed this particular issue)
ZC 1.5.4

When checking out using store credit funds and store credit funds exceed order total, a payment type is required even though the balance due is $0. Resulting in endless loop (from customer comment)

Use Store Credit funds minus one cent and selecting payment type completes order as expected.