InfoPath Dev

In IP 2003 I was able to use an expression like the following in the Conditional Formatting dialogue.

my:field1 = xdExtension:TestCF()

In InfoPath 2007, I get the error: InfoPath cannot open the selected form.

Are there extra steps to get this to work in IP 2007?

Notes

• TestCF returns a string. 
• This is a new blank form template

TIA for the help.