The argument type 'Map<String, dynamic>' can't be assigned to the parameter type 'String'

Question

someone help me for my issu , i can not solve my problem

fetchProducts() async {
    var _firestoreInstance = FirebaseFirestore.instance;
    QuerySnapshot _qn = await _firestoreInstance.collection('products').get();
    for (int i = 0; i < _qn.docs.length; i++) {
      _products.add(
      {
        "product-description" : _qn.docs.[i]["product-description"],
      }
      );
    }
  }