Why does a cell return #N/A only after I re-open the file?

The cell input line looks like this: =SUM(D6,D7,E11,D24,D26,D32,E36,D39,D40,D53,D62,D107,D110,E70,D73,D76,E116,D137,D142,E153,D162:D165,D173,D185,D187,D190,E195,D202:D206,E227,D253,E262,D266,D276,E292)
and it returns the correct value when I enter it and save the file. However, when I close the file and re-open it, the input line looks like this:
=NA()

Thanks in advance