Lookup not working - dimension used for lookup doesn't match any dimension of the source
I have 2 line items in the same module which are both in list format using the same subset list.
I would like 1 of the line items to be completed based on the entry made in the other.
I have created a mapping module using the same subset list as for the module and the line item (see attached screen shots).
But when I try and lookup from one to the other (formula shown in attachment) using the mapping, I get the "dimension used for lookup doesn't match any dimension of the source" error.
Any ideas what I am doing wrong here, as I am sure I had it working like this previously, but using an older list which has now been replaced.