Custom Field derived from Lookup not showing as List Item ID in SharePoint Designer


I'm not sure if this is an issue with the Custom Field or SharePoint Designer, but i thought i'd raise it here to see if anybody else has attempted to do this?
Using the 'Set a Workflow Variable' select Field Type "List Item ID", then attempt to populate this variable with a Custom Field derived from type "Lookup" / "SPLookupField", and it's not available on the list.