E10: Hide Description On Job Traveler Based On Part Name

On our job travelers, we'd like to hide the part descriptions for all parts except for a certain series of parts. These parts all start with 9UM. 


I've tried to hide the Part Description by going to the visibility option in the text box properties and setting up an expression. That expression is:


=Fields!PartNum.Value <> "9UM%"


From my understanding, this expression should hide all descriptions except for parts starting with 9UM. It's not working that way.


Can anyone confirm whether or not my expression is correct and, if not, any thoughts on what expression I need to have to hide part descriptions?