Created a GELI-setup using only a keyfile, partition to be mounted at boot.

When trying to set the secondary-key (key 1) using a passphrase incase the keyfile is lost, the following happens:

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42-gw# geli setkey -v -n 1 /dev/mirror/world
Enter new passphrase:
Reenter new passphrase: 
Calculating number of iterations...
Done, using 138302 iterations.
geli: Only already defined key can be changed when '-i' option is used.
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Note that the provider was attached at the time.

Fix: 

Workaround:

Setting the first key (key 0) that was used for attaching the provider with a keyfile, works:

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42-gw# geli setkey -v -n 0 /dev/mirror/world
Enter new passphrase:
Reenter new passphrase: 
Calculating number of iterations...
Done, using 80194 iterations.
Done.
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After that, setting the secondary key also works:

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42-gw# geli setkey -v -n 1 /dev/mirror/world
Enter new passphrase:
Reenter new passphrase: 
Done.
42-gw# 
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After that, I can set the primary key back to the keyfile.
How-To-Repeat: Create a GELI slice with only a keyfile, no password, to be mounted on boot, then try to set secondary key while the provider is attached.