Django-guardian: DoesNotExist: Permission matching query does not exist.

In my project I get a DoesNotExist error whenever I try to set permissions after a POST.

My models.py

class Project(models.Model):
    # Leader of the project
    project_name = models.CharField(max_length=200, unique=True)

    # The url for the stash repository 
    #project_repo = models.

    # Date the project was created
    date_created = models.DateTimeField(auto_now=True)

    # Person in charge of the project
    project_lead = models.ForeignKey(User, related_name='%(class)s_lead_set')

    # Group of members to use for permission checking
    members = models.ForeignKey(Group, null=True, default=None)

    class Meta: 
        permissions = (
            ('edit_project', 'Edit Project'),
        )

My views.py

@login_required
def create(request): 
    if request.method== "POST": 
        form = NewProjectForm(request.POST)

        if form.is_valid(): 

            # Create the new project
            project = Project(**form.cleaned_data)
            project.project_lead = request.user

            # Create a user group
            members = Group.objects.create(name = project.project_name)
            members.save()

            # Add the members group to the project
            project.members = members
            project.save()

            assign_perm('edit', members, project)
            members.save()