Thank you, everyone, for your prompt replies. I think I understand
now why it only *appeared* to be working, but if you pick the right
combination of numbers, it soon becomes apparent that it doesn't. I
did wonder, since it appeared to work for me but the colleague I did
it for reported that it did not work for her, so I suspected something
was up.

However, T.Valko's suggestion of =LOOKUP(1E100,A:A) does exactly what
I need, so essentially it's problem solved in this case. Thank you
for the information.

Steve