code: func (c *CertificateService) getIssuerFromLink(up string) ([]byte, error) {
  	if len(up) == 0 {
  		return nil, nil
  	}
  
  	log.Infof("acme: Requesting issuer cert from %s", up)
  
  	cert, _, err := c.get(up)
  	if err != nil {
  		return nil, err
  	}
  
  	_, err = x509.ParseCertificate(cert)
  	if err != nil {
  		return nil, err
  	}
  
  	return certcrypto.PEMEncode(certcrypto.DERCertificateBytes(cert)), nil
  }
docs: // getIssuerFromLink requests the issuer certificate