I'm having a little problem while defining a domain filter on a sale.order.form

I'm on a sale.order and I wanna update a domain filter for the field 'partner_invoice_id'. Reason is hiding the invoice address if it is the same as the partner id. So if partner_id == partner_invoice_id hide the field.

The error message is:   ValueError: Invalid leaf [6, u'!=', 6].  What is wrong with that?

This is the code:

<xpath expr="//field[@name='partner_invoice_id']" position="attributes">
						<attribute name="parent_id">id</attribute>
						<attribute name="readonly">1</attribute>
						<attribute name="context">{'show_address': 1,'default_type':'invoice','partner_id':partner_id}</attribute>
  						<attribute name="options">{"always_reload": True}</attribute>
						<attribute name="domain">
							['&amp;', ('type', 'in', ['default', 'invoice', 'contact', 'other']),
							 '&amp;', (bool(partner_id), '=', True),
							 '&amp;', (bool(partner_invoice_id), '=', True),
	 						 '&amp;', (partner_id, '!=', partner_invoice_id),	<!-- this doesn't work. If I remove this, my form will work	
							 '|', ('id', '=', partner_id), ('parent_id', '=', partner_id)]
						</attribute>
						<attribute name="widget">selection</attribute>
					</xpath>