Find last value in a column of formula derived values.

I'm certain there is a simple explanation; I have a column (E) of values where each cell has the following example formula:='[Workbook1.xlsx]Sheet1'!C103 and the only change to each cell as you go down the column is the workbook reference. Simple. I also have at the top of the sheet, a formula to return the last value in this column:

=(INDEX(E7:E106,MATCH(9.99999999999999E+307,E7:E106))))

Also simple. However, it doesn't work. If I manually input the information in column E, it works. I therefore assume that this function will not work with values derived from formula. Any ideas anyone?