Contribution amount when set to £0 hides payment block

When entering an empty or 0.00 value into the (Other Contribution amount) textbox (or eg entering an amount and then removing it) on a Contribution page which is setup to use Memberships, the Payment fields (Payment Options, Credit Card info, Billing Name and Address etc) are hidden. Despite there being a total value which is greater than 0, because there is a value on the Membership price.

This works fine on a normal Contribution page, because the Contribution amount is registered as a separate option.

Steps to reproduce on the test site:

1. Create Membership type with a Minimum fee
2. Create a Contribution page to support this Membership type
3. In 'Amounts' tab, select the option 'Allow other amounts'
4. Visit Contribution and you will see the Payment/Billing fields, enter 0.00 into the (Other amount) textbox and these fields will disappear