The following code compiles:

class Foo : Object {
    public string my_property { default = "none"; }
}

void main () {
    new Foo ();
}

but produces the runtime critical error:
GLib-GObject-CRITICAL **: g_object_class_install_property: assertion 'pspec->flags & (G_PARAM_READABLE | G_PARAM_WRITABLE)' failed

The same for:

class Foo : Object {
    public string my_property {}
}

void main () {
    new Foo ();
}