Problem in configuration>checkout>Default order status

[joo$st3]

VM 2.0.20b and Joomla 2.5.11
Iam trying to get Artio VM invoice to work, followed the following instructions:
To prevent customer from getting also VM2 native invoices, go to VirtueMart -> Configuration ->Checkout -> and make sure section "Default Order Status to send an invoice" is empty.

But whatever I try I cannot empty the 'default order status to send an invoice'. According to Artio this should be a VM bug. Hereunder their answer:
this is probably bug in VirtueMart. My version of VM does the same thing, If I try to set empty status and save settings, it switches to previous setting.

[jenkinhill]

See http://forum.virtuemart.net/index.php?topic=115202.0