Need to add a restriction to a current formula, but not quite sure how to do it!

[SpeckPumps]

Hi everyone. I am currently using Cyrstal Reports 2008 Version 12.3.0.601. It is important for you to understand that I am a novice at this, so please forgive my stupidity. I currently have a Formula that does this:

IF {ARTRNDETAIL.NetSalesValue} = 0
THEN 0
ELSE ({#GrossProfit}/{#NetSalesValueYTD}*100)

It works as needed, but I need to create another formula that does the same thing as above, but I need to add this restriction to it:

{ARTRNDETAIL.TrnMonth} = Maximum({?Period})

I tried to do it in this manner:
{ARTRNDETAIL.TrnMonth} = Maximum({?Period}) AND
IF {ARTRNDETAIL.NetSalesValue} = 0
THEN 0
ELSE ({#GrossProfit}/{#NetSalesValueYTD}*100)

But it doesn't work. (States "A Boolean is required here" but I'm not sure where or how.) It's probably obvious to many of you, but I cannot seem to figure it out. Any help is greatly appreciated. Thanks in advance.

[Ned Pepper]

The error "A Boolean is required here" is returned because a formula is similar to a database field in that is must be of 1 and only 1 data type. The statement {ARTRNDETAIL.TrnMonth} = Maximum({?Period}) returns a Boolean (true/false) value, while the if-then-else portion of the statement would return a numeric value. Not sure what you are attempting to do here, but make the code work you could try changing to

IF {ARTRNDETAIL.TrnMonth} = Maximum({?Period}) THEN
    IF {ARTRNDETAIL.NetSalesValue} = 0  THEN
        0
    ELSE ({#GrossProfit}/{#NetSalesValueYTD}*100)
ELSE
// when {ARTRNDETAIL.TrnMonth} <> Maximum({?Period})
    0