Comment 9 for bug 556549

        if (EC_FillParams(arena, &pubKey->u.ec.ecParams.DEREncoding,
                    &pubKey->u.ec.ecParams) != SECSuccess) {
            crv = CKR_DOMAIN_PARAMS_INVALID;
            break;
        }

from pkcs11.c in NSS, 1629-1634 in GetPubKey

which returns

cleanup:
    if (!params->cofactor) {
        PORT_SetError(SEC_ERROR_UNSUPPORTED_ELLIPTIC_CURVE);
#if EC_DEBUG
 printf("Unrecognized curve, returning NULL params\n");
#endif
    }