Tengo un problema cuando uso el métodoX509Store.Certificates.Find

public static X509Certificate2 FromStore(StoreName storeName, 
          StoreLocation storeLocation, X509FindType findType, string findValue)
{
    X509Store store = new X509Store(storeName, storeLocation);
    store.Open(OpenFlags.ReadOnly);
    try
    {
        //findValue = "7a6fa503ab57b81d6318a51ca265e739a51ce660"
        var results = store.Certificates.Find(findType, findValue, true);

        return results[0];                
    }
    finally
    {
        store.Close();
    }
}

En este caso, el método Find devuelve 0 resultados results.Count == 0), pero si pongo findValue como constante, el método encuentra el certificado.

public static X509Certificate2 FromStore(StoreName storeName, 
           StoreLocation storeLocation, X509FindType findType, string findValue)
{
    X509Store store = new X509Store(storeName, storeLocation);
    store.Open(OpenFlags.ReadOnly);
    try
    {         
        //findValue= "7a6fa503ab57b81d6318a51ca265e739a51ce660"
        var results = store.Certificates.Find(findType, 
                              "7a6fa503ab57b81d6318a51ca265e739a51ce660", true);
        return results[0];
    }
    finally
    {
        store.Close();
    }
}