[CentOS] cryptsetup

On a just-built C 7 box, I've got this: boot, boot_efi, /, swap, and
export. Now, the last three are encrypted. For /, I'm trying to add an
escrow key. df -h shows me that / is /dev/dm-1. However, when I use
cryptsetup luksAddKey /dev/dm-1, it just comes back, without prompting me
for a password.

What am I missing?

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