Error msg "Cannot read property 'indexOf' of undefined"

Receiving an error msg that reads, "Cannot read property 'indexOf' of undefined", but I'm not sure why it's undefined. Here's the code that's causing the issue:

if (req.user) {
    swig.setFilter('userLikes', function(journeyPassportID) {
      if (req.user.journeys_liked.indexOf(journeyPassportID) != -1) {
        return 'liked';
      }
    });