One Question at the Checkpoint

Deep in a border zone the road splits into two tunnels: one leads to the crossing, the other dead-ends in a minefield. A lone sentry knows which is which — but sentries posted here come in two kinds, those who answer every question truthfully and those who answer every question falsely, and you cannot tell the kinds apart. You are permitted exactly one yes/no question. What do you ask to find the safe tunnel?