Pivot Table - Reference is not valid

I have a list in a file name abc.xls. I have defined the list with a
name say XYZ.
I have another file Summary.xls with a pivot table that I'm using to
give me a summary of that list.
The 'XYZ' which is the defined name of the actual list in abc.xls file
works when both the files are open.

When I open the Summary.xls file, first message says "Reference is not
valid" and second message says "Cannot find "abc.xls file".

If I first open the abc.xls file and then the summary.xls, then it
works and not viceversa.

Please update.

Thx.