How to properly type dbus dicts

I'm trying to build a simple C++ app that can talk to NetworkManager via DBus, query the connection state, and add new wifi connections.

However, I seem to be running into an issue when querying information about a network connection which has a type of `a{sa{sv}}`. I've been using the `dbus-cppc` as a starting point and it says the return type of org.freedesktop.NetworkManager.Settings.Connection.GetSettings should be:
`typedef std::vector<std::pair<std::string, std::vector<std::pair<std::string, core::dbus::types::Variant>>>> ResultType;`

But I'm getting a runtime error when running my app:
terminate called after throwing an instance of 'std::runtime_error'
  what(): Mismatch between expected and actual type reported by iterator:
  Expected: string
  Actual: dictionary_entry

The ResultType provided looks correct to me, and I've searched around a but could't find any of the examples that have a type like this. What's the proper ResultType for a dict within a dict? A stripped down example of what I'm working with can be found here:
https://gist.github.com/brentalanmiller/e774b3bbb53e9f08f22a9fe05f95b0aa