Can't start a new workflow on pages with no parent

Steps to reproduce:

1. Install Publication Workflow Application
2. Create and add users for the required groups: Contributors, Moderators, Publishers, Viewers and Commenters
3. Add 'PublicationWorkflow.PublicationWorkflowPanel' to a left/right panel
4. Go to Administer Wiki > Look & Feel > Navigation Panel and add 'PublicationWorkflow' to 'Navigation' section
5. Create a new Publication Workflow Config page from Template (and add a default Draft Space)
6. Login with a Contributor
7. Create (from the Home/Main page) a new page with some content 
8. While on that page, in the Workflow panel, click 'Advanced options' and select a Publish Location (e.g. Sandbox)
9. Click 'Start workflow'

Expected results: A message saying "Current status: Draft" appears, and under it the Submit to moderation button, in the left panel.

Actual results: The message "Published page already exists." appears.