Separate Membership Payment Broken

On a payment page with Payment Amounts::Allow Other Amounts, Membership Amounts and Separate Membership Payment enabled, if you attempt to process the page with the "Donation"/Other Amount box empty you get an error "Failed to update contribution in database"
Steps to Reproduce:
In Sandbox::Master:

• Enable the Other amount box on the sample membership page,
• Enable the Separate Membership Payment box.
• Attempt to renew your membership without donating