if (valueItr == null) {
          valueIterators.put(key, valueItr = sampleValuesIterator());
        }
        entries[i] = mapEntry(key, valueItr.next());
      }
      return multimapGenerator.create((Object[]) entries).keys();
    }

    private Iterator<V> sampleValuesIterator() {
      return ((TestMultimapGenerator<K, V, M>) multimapGenerator.getInnerGenerator())