Reporting Services problem

[Rainbow River]

I have a problem in reporting services.
In the report, I have field1 (int) , field2 (char) and a group based on field1.
Now, I want Avg function for the group.
Something like:
if field2="C" then exclude it from the Avg function.
The function I wrote is:

Avg(iif((Fields!field2.Value="C"),0,Fields!field1. Value))

But I got #Error in the group field when I preview it.
Can someone help me?
Thanks!

[MAK]

may be its doing a divide by zero. check if it doesnt return zero when you add all the rows.