Workflow Rule Valid on Multi Select Field Value

I am trying to make an Opportunity workflow rule valid for java script condition Valid=(oppo_prod=='trapp'). Oppo_prod is a multi select field with 35 options. When it gets to this point in the workflow, it does not display. If I click change and then save the opportunity (without making any changes) it then appears. How can I make the workflow validate without having to do this extra step?