All registration or other fees provided for in this chapter shall constitute a lien against the vehicle for which the fees are payable unless otherwise provided in this section until such time as they are paid as provided by law, with any accrued penalties. The county treasurer may perfect a security interest in a vehicle for the amount of such fees as provided in section 321.50. If the lien is not perfected as provided in this section, the lien shall not be valid against a bona fide purchaser of the vehicle without actual notice to the purchaser.