Excel 2003 Hiding #DIV/0 in Data/Subtotal/Avg result

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Subject: Excel 2003 Hiding #DIV/0 in Data/Subtotal/Avg result
Posted by:  Karl Perry (kaNOPer…@SPAMcablespeed.com)
Date: Sun, 25 Jun 2006

I have a worksheet organized as:
WeekDay / WoodShop / Spa / Pool
Monday /    / 1 / 4
Tuesday/    / 2 / 2
Wednesday/  / 1 / 4

I used the Data/Subtotal/Average functionality to determine the average
number of participants in each of the events over the period of time, and
the Woodshop average's cell displays "#DIV/0".

I understand that dividing 3 by 0 is an error, but I want Excel just to
leave the cell blank.

I've tried conditional formatting using the Is Formula / =iserror as
suggested in Help, but Excel is ignoring this.

Is there any way to make Excel do what I want?