can't parse delegation from proof

[gm0stache]

what do i want to do:

create a client to programmatically upload '.car' files to web3.storage.

what i did:

1. created a web3.storage account
2. installed the w3 CLI and signed in (w3 login)
3. created a key (w3 key create > private.key)
4. created a delegation for the key (w3 delegation create -c 'store/*' -c 'upload/*' [key DID] -o proof.ucan)

the problem encountered:

when trying to parse the proof.ucan with

package main

import "github.com/web3-storage/go-ucanto/core/delegation"

func main() {
	proofDel, err := delegation.Extract(proof)
}

i encounter get this error: missing root CID in delegation archive

am i doing something wrong here?

i already tried to pass the account DID of mine (did:mailto:...) instead of the private key DID (created in step 3.), even tough it didn't make sense to me. this failed too.

=> is the private key DID the right one to pass to the delegation in step 4?

thanks a lot for all you work and greetings from germany!