Server side delegated credentials

[Akatsukikain]

I'm trying to use server side delegated cerentials with Fizz using the following command but I get an error, also I've got some questions:
./fizz s_server -delegatedcred deleg -cert delegation_cert.pem -key p256_dc_key.pem
--> E0616 18:31:50.221060 24958 FizzServerCommand.cpp:977] Credential parsing failed: underflow

Why doen't it work ? I used a valid credential that I got from fizz gendc command

Also I get that for the cert it's going to be the delegation certificate but what about the -key is it the key of the delegation_cert or the credential ? Or is it both in that case isn't it weird since the whole point of delegated credentials is to make sure our cert's private key is safe and secure ?

Thanks in advance
Hala