Using TTable's Lookup Method ?

Can anyone tell me what I am doing wrong here?
I c
keep getting the same answer either way

Note: LastLogIn is a DateTime Stamp

var
 V: Variant;

V:= tblAccounts.Lookup('UserName; Password', VarArrayOf([U,P]),'LastLogIn');
if not (VarType(V) in [varNull]) then
    ShowMessage('Account exist!')
   else
    ShowMessage('Account does not exist');