Hi

You're quite close though cells A2:A4 in your example are empty which leaves you to a REf-error.
So the first thing to remember, if the range you refer to contains a reference which doesn't exist you will end up with the error!
Therefore just fill the names of the desired sheets, e.g.:
A2: Sheet1
A3: Sheet2
Change your formula to this range and it will do the trick.