ZFS hot spare not working

I have the fallowing ZFS test pool setup in my vm:

    NAME        STATE     READ WRITE CKSUM
    pool        DEGRADED     0     0     0
      mirror-0  DEGRADED     0     0     0
        sdb     ONLINE       0     0     0
        sdc     ONLINE       0     0     0
    spares
      sdd       AVAIL

I a trying to test the ability to replace a drive that becomes unavailable automatically in a zfs pool. For example if for some reason the drive is removed from the system the spare dive could take its place immediately upon the drives removal. I have configured a spare drive and from the reading I have done, the way I understand it is that this should accomplish my goal. I have also set the autoreplace flag to on to tell zfs to replace any bad drive if detected. When remove a drive from the vm, say sdb and recheck the status of zfs it looks like the fallowing:

    NAME        STATE     READ WRITE CKSUM
    pool        DEGRADED     0     0     0
      mirror-0  DEGRADED     0     0     0
        sdb     UNAVAIL      0     0     0
        sdc     ONLINE       0     0     0
    spares
      sdd       AVAIL

The sdb device becomes unavailable but is never replaced by the spare. Is there a way i can get my use-case to work?

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