Query won't work in new database...Help!

OK, I have a second version of a database that I am doing because I am much,
much more proficient now. I started over from scratch because I hadn't done
the relationships and stuff right the first time around. I am trying to
recreate my queries and stuff and am running into a problem with my
attendance tracking query.

The fields I am using for my query a First_Name, Last_Name, Contact_Name
(which is a formula to combine the first and last name) and the leave
remaining formula. Here is the formula I am using to calculate my leave
time: VRemaining:
First([Vacation_Given])-Nz(Sum(IIf([Reason]='Vacation',[Hours_Taken],0)),0).

Now, I copied this formula from my original database where it worked fine.
However, when I try to run it on the new database I get the following error:
"You tried to execute a query that does not include the specified expression
'First_Name' as part of an aggregate function.

I have no idea what that means and have tried a thousand different things to
troubleshoot and work around it. Nothing has worked. The only difference
between this and my original database is that in the original one I had
[Employee_Name] instead of [Contact_Name] and I figure it has something to do
with that, but for the life of me I cannot figure out what. Can someone help
me? If needed I can post the SQL code for both the original and the new.

Thanks!
Brooke