How to Stop #Value appearing?

Hi

This is my first post so hope its in the right place. I've searched the forums and cant find an answer to this query.

I'm working on a sheet to automatically calculate rates based on a cost book of items from a drop down list. The final column is a simple =D13*E13 formula. However this displays the #Value error message in the cells which do not yet have rates selected - is there any way of getting rid of this - or a better way to handle my formatting?

Thanks for any help

Paul