ca-set-passphrase -- what is this?

At https://wiki.mikrotik.com/wiki/Manual:System/Certificates there is no description for ca-set-passphrase command.

Does this command encrypt the private key of CA?

I’ve create the CA

/certificate
add name=my-ca common-name=my-ca trusted=yes key-usage=key-cert-sign,crl-sign
sign my-ca

Then I try to Set a Passphrase for CA:

ca-set-passphrase my-ca ca-passphrase=MyStrongPassphrase
failure: Invalid CA passphrase!

What I missed?