Delete Sheet before Importing Sheet calculation issue

I have a summary sheet that reference two sheets that are imported each month. (CAN DATA and USA DATA)

The next month I delete these two sheets before importing current month's details. But when I do that it screws up my references.

Here's an example of a reference with previous months' imported text file details:

='USA DATA'!AQ12+'CAN DATA'!AQ12

When I delete the sheets and import current month's details, my reference suddenly looks like this:


=#REF!AQ12+#REF!AQ12

What can I do to prevent the summary sheet from messing up my original references?