How to import SSH keypair into 6.4?

I'm trying to import an openssh keypair into a 6.4.x FortiAnalyzer for use in backup via SCP.  I did the typical ssh-keygen on the remote side.  On the FortiAnalyzer side, I:

config system certificate ssh

  edit "backup"

and I was able to successfully import the private key as is by simply "set private-key" and then quoting the text as it comes out of the key file on the host.  I'm attempting to import the public key via "set certificate" but it will not accept it in native format (cat id_rsa.pub), RSA format (ssh-keygen -f id_rsa.pub -e -m pem), or openssl pem format (openssl rsa -in id_rsa -pubout -out id_rsa.pub.pem).  All three attempts just return:

Invalid certificate. Command fail. Return code -61