case "ANSWER_TEXT": {
      const { index, answerIndex, data } = action;
      const { answer } = data;

      return Immutable.fromJS(state)
        .mergeIn(["ys_list", "list", index, "my", answerIndex], answer)
        .toJS();
    }

Stack trace:

immutable.es.js:2202 Uncaught TypeError: Cannot merge into non-data-structure value:
    at mergeWithSources (immutable.es.js:2202)
    at eval (immutable.es.js:2271)
    at updateInDeeply (immutable.es.js:2052)
    at updateInDeeply (immutable.es.js:2065)
    at updateInDeeply (immutable.es.js:2065)
    at updateInDeeply (immutable.es.js:2065)
    at updateInDeeply (immutable.es.js:2065)
    at updateInDeeply (immutable.es.js:2065)
    at updateIn (immutable.es.js:2030)
    at Map.mergeIn (immutable.es.js:2271)