Array formula vanishing

I have a spreadsheet with some array formula (to create a unique distinct list of names from multiple entries & it works just fine). But the next time I open the spreadsheet again, where the result of the array formula should be 0, the formula just disappears leaving me with an empty cell. Where the formula does return a name, the formula is still there.
Is this a bug or is there something I can do to stop the formula just vanishing?

I’m saving as a .xls M$ Excel 97-2003 file. Using Version 4.2.3.3, on MS Windows Vista Business.

The array formula
{=INDEX($B$75:$B$323,MATCH(0,COUNTIF($S$75:S93,$B$75:$B$323),0))}