Data Validation - source evaluates to an error - why?

I've had to start my workbook from scratch. I'm using two excel books, one of which contains all the source data for look-ups and the other for inputting records (it's necessary to do this as the reference data will account for 90% of space, plus the code was all over the place).

I've followed instructions on how to create a data validation from another workbook by this method http://www.contextures.com/xlDataVal05.html. Both are updated, saved and open. There is no formatting except for cells in the data inputting book being merged. But I keep getting the message "The source currently evaluates to an error".

I do get the arrow next to the merged cell that is to be the dropdown but that's it (and I've tried it with an unmerged cell).

I've tried everything - starting anew, using path names, simple names but I still keep getting the error. Can anyone offer advice?

Got it now - didn't use ' character on either side of the filename Where's the embarrased smiley?