Sorting data linked to other spreadsheets
 

Everytime I sort data on my main spreadsheet the spreadsheets that are linked
get a lot of #REF! errors. The formulas in my other spreadsheets are all
similar to:

=IF('Enrollment Log'!$B491=4099, 'Enrollment Log'!$H491, " ")
=IF('Enrollment Log'!$B492=4099, 'Enrollment Log'!$H492, " ")

When the data is re-sorted if there was originally data the met the above
criteria in cell B491 but it moved then that cell will produce an error of
#REF! Is there a way I can fix this?