How do I repair an encrypted sparse disk image from the command line?

I have tried:

hdiutil repairVolume my.dmg.sparseimage

But I get the error:

Ownership of the affected disk is required

The sparseimage I am trying to repair is owned by me, and I have R/W privileges. What is going on? How do I repair the image?

(By the way, I am trying to repair because the automatic verify on mount fails to mount read-write, saying there is a problem it can't repair, so I want to try it manually before I mount the disk.)