Remove users from Portal programmatically

I'm trying to bulk remove users from a portal. In the absence of a fluent api I'm trying

portal.removeFromPortalUsers(portaluser)

and

user.removeFromPortalUsers(portaluser)

in both cases I'm getting an error which only differs depending on what object is calling the method. In the case of portal I get:

[v.3.1 Sep 20 2014 14:06:21] Validation failures: Validation failure for com.avoka.fc.core.entity.PortalUser.portal: \"portal\" is required.

and invoking on user I get:

[v.3.1 Sep 20 2014 14:06:21] Validation failures: Validation failure for com.avoka.fc.core.entity.PortalUser.user: \"user\" is required.

Is there a way around this?

Full code below:

portal = new com.avoka.fc.core.dao.PortalDao().getPortalByName('My Portal')

users = portal.getPortalUsers()

outsiders = users.findAll { ! it.getUser().isActive() }



outsiders.each { pu ->

	u = pu.getUser()

	u.removeFromPortalUsers(pu)

}