The findValue parameter for the X509Certificate2Collection.Find(X509FindType, object, bool) method must be a string representing the issuer distinguished name of the certificate. This is a more specific search than that provided by the X509FindType.FindByIssuerName enumeration value. Using the X509FindType.FindByIssuerDistinguishedName value, the X509Certificate2Collection.Find(X509FindType, object, bool) method performs a case-insensitive string comparison for the entire distinguished name. Searching by issuer name is a less precise search.