Free memberships are broken

Steps to reproduce (as performed on d46.demo.civicrm.org)

1) create a free membership type (member dues = 0)
2) create a minimalist contribution page
enable your new free membership type on the contribution page
untick 'real time financial transactions' and any contribution options
3) fill out the form, which should only display the free membership type and email address field.

Expected result:
I expect a completed zero value contribution, a membership and an activity to be recorded against the contact.

Actual result:
I see a pending zero value contribution, an activity and *no* membership on the contact.

The following errors are thrown:

Notice: Undefined offset: 1 in CRM_Member_BAO_Membership::postProcessMembership() (line 1339 of */sites/all/modules/civicrm/CRM/Member/BAO/Membership.php).

Notice: Undefined index: payment_processor in CRM_Contribute_Form_Contribution_Confirm->postProcess() (line 682 of */sites/all/modules/civicrm/CRM/Contribute/Form/Contribution/Confirm.php).