View Purchase Order from Invoice Workbench

I am in the Invoice Workbench APXINWKB. I query a PO matched invoice. In my toolbar menu, I see a menu item "View Purchase Order", but this is grayed out. I would like to View the PO, but I cannot as the menu is grayed out. How can I get this menu item to work?

Please Note that from the Distributions window I see another button "View PO". When I click here it takes me to the PO Shipments screen. However, I would like to go to the PO Lines screen and not Shipments.