g_dbus_proxy_new_for_bus_sync: assertion 'g_variant_is_object_path (object_path)' failed

The code following code should retrieve systemd-logind IdleSinceHint property exposed on DBus

/*
 * Compile with:
 *   gcc -Wall print_user_idle_time.c -o print_user_idle_time `pkg-config --libs gio-2.0 --cflags`
 */

#include <gio/gio.h>

static void
print_user_idle_time (GDBusProxy *proxy)
{
    gchar *property = "IdleSinceHint";
    GError *error = NULL;
    GVariant *ret;
    guint64 user_idle_time;

    ret = g_dbus_proxy_get_cached_property(proxy, property);
    if (!ret) {
        g_dbus_error_strip_remote_error (error);
        g_print ("IdleSinceHint failed: %s\n", error->message);
        g_error_free (error);
        return;
    }

    g_variant_get (ret, "(^ao)", &user_idle_time);
    g_print("%lu\n", user_idle_time);
    g_variant_unref (ret);
}

int
main (int argc, char *argv[])
{
    GDBusProxy *proxy;
    gchar *name = "org.freedesktop.login1";
    gchar *object_path = "org/freedesktop/login1";
    gchar *interface_name = "org.freedesktop.login1.Manager";
    /* Create a D-Bus proxy */
    proxy = g_dbus_proxy_new_for_bus_sync (G_BUS_TYPE_SYSTEM,
                                           G_DBUS_PROXY_FLAGS_NONE,
                                           NULL,
                                           name,
                                           object_path,
                                           interface_name,
                                           NULL, NULL);
    g_assert (proxy != NULL);

    print_user_idle_time (proxy);

    g_object_unref (proxy);

    return 0;
}

but when it is runned it fails on assertion g_assert (proxy != NULL); with error

(process:9059): GLib-GIO-CRITICAL **: 17:29:07.245: g_dbus_proxy_new_for_bus_sync: assertion 'g_variant_is_object_path (object_path)' failed
**
ERROR:print_user_idle_time.c:44:main: assertion failed: (proxy != NULL)

What can be the problem? Thank you

The issue you are seeing is that object paths must start with '/' so it's /org/freedesktop/login1.

In addition to that, the property GVariant handling line looks out of place. This should be enough:

user_idle_time = g_variant_get_uint64 (ret);