Need to update a 'Lookup' field from another list.

[#1]

I have two lists on the site. The one list has a reference number that refers to the other list. I am trying to put together a workflow with a query that gets all items with the related reference from the list but am having some challenges in updating the lookup field. I am getting the right ID's from the list but when i attempt to update the item all I get is "Error updating a list item" With Outcome "Unknown error".

I am pretty certain my problem is how i am building my variable is use to update the Lookup field.

[#2]

Ok, Managed to get my problem solved, the issue was in how I was building up the variable. Found an answer after some more searching.

The specified item was not found.

[#3]

Thanks Riaan this worked