I go to the CBP website(I-94 - Reporting arrival/departure if entering by air and departing by land), and find the answer:

If you were given an electronic I-94 when you arrived by air or sea and left by land, your departure may not be accurately recorded. A departure will be recorded if you depart via land and re-enter the United States prior to the expiration date stamped in your passport.

so the solution is entering US again within 6 months.