tls with relayd (on 5.7) and key without password

[Comète]

Hi,

my tls key has no password and i already use it for other stuff, so i try to 
enable TLS with relayd like this:

http protocol "http_tls" {
    tls tlsv1
    tls ca key "/etc/ssl/private/mydomain.org.key" password "" 
    tls ca cert "/etc/ssl/mydomain.org.crt"
}

relay transptls {
    listen on $ext_addr port 443 tls
    protocol "http_tls"
    transparent forward with tls to "127.0.0.1" port http
}

but i get this error:

startup
socket_rlimit: max open files 1024
socket_rlimit: max open files 1024
relay_load_certfiles: using ca certificate /etc/ssl/mydomain.org.crt
socket_rlimit: max open files 1024
socket_rlimit: max open files 1024
relay_load_certfiles: using ca key /etc/ssl/private/mydomain.org.key
/etc/relayd.conf:24: cannot load certificates for relay transptls
no actions, nothing to do
ca exiting, pid 29173
pfe exiting, pid 19946
ca exiting, pid 3806
ca exiting, pid 24689
hce exiting, pid 32289
relay exiting, pid 22936
relay exiting, pid 25790

So, is it possible to use a tls key without password with relayd ?

Thank you

Morgan