Java Security

I'm trying to work with Digital IDs from Verisign for S/MIME. I must however be doing something very wrong as I'm not getting very far.

I downloaded from Verisign the recipient's Certificate/Digital ID.
So far so good, I can use CertificateFactory to load in the file.

Problem I'm having is that I can't verify the Certificate. The error message is:

Exception in thread "main" java.security.SignatureException: Signature does not match.
        at sun.security.x509.X509CertImpl.verify(X509CertImpl.java:400)
        at sun.security.x509.X509CertImpl.verify(X509CertImpl.java:363)
        at TestCertificate.main(TestCertificate.java:23)

Here's the code snippet I'm using:

        String filename = "base64.cer";
        CertificateFactory cf = CertificateFactory.getInstance("X.509");
        FileInputStream fis = new FileInputStream(filename);
        BufferedInputStream bis = new BufferedInputStream(fis);
        Certificate cert = null;
        while (bis.available() > 0) 
        {
            cert = cf.generateCertificate(bis);
            System.out.println(cert.toString());
        }
        
        PublicKey key = cert.getPublicKey();
        cert.verify(key);

Why wouldn't a certificate verify?