Using the snippet generator to generate the code for a reverse build trigger generates code like this:

properties([pipelineTriggers([upstream(threshold: <object of type hudson.model.Result>, upstreamProjects: 'foo')])])

The Result object needs to be displayed as e.g. hudson.model.Result.SUCCESS.

I'm not sure in which component this needs to be fixed; I've filed against core (which implements ReverseBuildTrigger) and workflow-job-plugin (which defines pipelineTriggers).