Unable to parse EC private key in PKCS#8 format

I'm trying to use dkimpy to sign messages from the commandline. I have an EC key generated with openssl which is valid (it's in use for HTTP TLS and other purposes without issues), which openssl dumps as follows (private data omitted...):

$ openssl ec -in fqdn.key -text
read EC key
Private-Key: (256 bit)
priv:
    [...]
pub:
    [...]
ASN1 OID: prime256v1
NIST CURVE: P-256
writing EC key
-----BEGIN EC PRIVATE KEY-----
[...]
-----END EC PRIVATE KEY-----

openssl can successfully parse it with ASN1:

$ openssl asn1parse -in fqdn.key
    0:d=0 hl=2 l= 119 cons: SEQUENCE
    2:d=1 hl=2 l= 1 prim: INTEGER :01
    5:d=1 hl=2 l= 32 prim: OCTET STRING [HEX DUMP]:E...
   39:d=1 hl=2 l= 10 cons: cont [ 0 ]
   41:d=2 hl=2 l= 8 prim: OBJECT :prime256v1
   51:d=1 hl=2 l= 68 cons: cont [ 1 ]
   53:d=2 hl=2 l= 66 prim: BIT STRING

I've tried with this file, as well as explicitly telling openssl to convert it to PKCS#8 format:

$ openssl pkcs8 -in fqdn.key -topk8 -nocrypt -out fqdn.key.pkcs8

$ openssl asn1parse -in fqdn.key.pkcs8
    0:d=0 hl=3 l= 135 cons: SEQUENCE
    3:d=1 hl=2 l= 1 prim: INTEGER :00
    6:d=1 hl=2 l= 19 cons: SEQUENCE
    8:d=2 hl=2 l= 7 prim: OBJECT :id-ecPublicKey
   17:d=2 hl=2 l= 8 prim: OBJECT :prime256v1
   27:d=1 hl=2 l= 109 prim: OCTET STRING [HEX DUMP]:3...

However, using dkimsign with either file as the private key results in an "unparsable private key" error. I'm calling it as:

$ dkimsign s1 fqdn <keyfile_path> < <rfc822_message_file>

When testing like this, I'm not sure what to use as the selector - I've tried various things, but it doesn't seem to be getting far enough for that to matter - it's dying in the ASN1 parsing function.

With the original file, it gives:

Unparsable private key (are your sure it is encoded in PKCS#1 or PKCS#8 standard ?):
Unexpected tag (got 04, expecting 30)

With the PKCS#8 format file, it gives:

Unparsable private key (are your sure it is encoded in PKCS#1 or PKCS#8 standard ?):
Unexpected tag (got 06, expecting 05)

I realize it's not ideal that I've elided the actual domain name in use here. If you like, I can provide it.

The only thing "unusual" about my setup that I can think of is that it's an EC key, rather than RSA. Is that supported?

Any ideas on what's wrong here?

Thanks,

Charles