PTC Navigate and SiteMinder

We got Navigate and SSL working, but when we turn on SSO tool Siteminder, we get the following error:

Wrapped javax.net.ssl.SSLHandshakeException: sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target Cause: sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target

Just wondering if anyone else is using Siteminder with Navigate. Many thanks in advance!