Parsing PKCS7 using bouncy castle

Question

I have received a p7b file from a client which should contain a certificate chain and am trying to extract the certificates from it using bouncycastle.

I am running the latest 1.55 version of bouncy castle and am using the code you see everywhere for this:

        byte [] content = ...;
        CMSSignedData data = new CMSSignedData(content);
        Store certStore = data.getCertificates();
        SignerInformationStore signerInfos = data.getSignerInfos();
        Collection<SignerInformation> signers = signerInfos.getSigners();
        List<X509Certificate> result = new ArrayList<X509Certificate>();
        for (SignerInformation signer : signers) {
            Collection<X509CertificateHolder> matches = certStore.getMatches(signer.getSID());
            for (X509CertificateHolder holder : matches) {
                result.add(new JcaX509CertificateConverter().setProvider("BC").getCertificate(holder));
            }
        }
        return result;

The problem is that the signers collection is empty.

Note that I was able to extract them successfully using openssl:

openssl pkcs7 -print_certs -in test.p7b -out test.pem

But I rather have a clean java-based solution. Am I missing something? Is the CMSSignedData not being constructed correctly?