geli: Provider zvol/mypool/gelizvol is invalid.

[Himan Zero]

Dear team,

I used to follow the section "18.12. Encrypting Disk Partitions" to create
a geli encrypted zvol in my FreeNAS server under FreeNAS version 9.2.1.5.
It works fine. But some day, when I upgrade the system to 9.2.1.8, I
suddenly found I could not attach the encrypted zvol anymore. What ever I
do, system shows the following message:

geli: Provider zvol/mypool/gelizvol is invalid.

I can replay the error in following steps under version 9.3 stable:

(Suppose my zpool is named as 'mypool' and I have created a zvol namely
'gelizvol'.)
Step one - initialize geli on gelizvol:
      sudo geli init /dev/zvol/mypool/gelizvol
      (press Enter twice to give a blank password)
Step two - attach the gelizvol
      sudo geli attach zvol/mypool/gelizvol
      (press Enter to give a blank password)

Then the system shows the error message:
      geli: Provider zvol/mypool/gelizvol is invalid.

I have tried many times in 9.2.1.5 and such operation will complete without
error but when I go back to the newer version, I constantly get such error
message and cannot get my geli encrypted zvol back.

I'm not sure whether you could help me out?

Many thanks!

Himan Zero