I’m mystified why the following Dart code generates a runtime error:

class Store<ID, T> {
  Store({Map<ID, T> values = const {}}) : _cache = values;

  final Map<ID, T> _cache;

  void put(ID id, T value) {
    _cache[id] = value;
  }
}

...
final foo = Store<int, Store<int, String>>();

// Error: TypeError: 7: type 'int' is not a subtype of type 'Never'
foo.put(7, Store<int, String>());

This isn’t an issue if I use a Map directly like this:

final foo = Map<int, Map<int, String>>();
foo[7] = Map<int, String>());

Is there something about Dart generics I need to understand to make Store work?