The return type ‘dynamic’ isn’t a ‘Map?’ from let

I have the same func in different files, but here i got an error

    Map<String, dynamic>? getJson(String key) =>
      _sharedPreferences.getString(key)?.let((it) => jsonDecode(it));

 The return type 'dynamic' isn't a 'Map<String, dynamic>?', 
as required by the closure's context.dartreturn_of_invalid_type_from_closure