This Hurtigruten trip I'm on, they're desperate to fill as many places as they can in a cold, damp, rainy November. In fact, for a couple of days last week, the crew outnumbered the passengers 41-38. So anyway, they're offering a northern lights "guarantee" in November and December, in that if the lights don't show then they'll give you a free trip in spring next year.
The bad news is, I won't be getting a free trip...
The good news is left as an exercise for the reader.