Safari number input stepUp/stepDown不起作用，值为空

<input type="number" id="myNumberInput" value="0">
<button id="stepUpButton">增加</button>
<button id="stepDownButton">减少</button>

<script>
  var numberInput = document.getElementById("myNumberInput");
  var stepUpButton = document.getElementById("stepUpButton");
  var stepDownButton = document.getElementById("stepDownButton");

  stepUpButton.addEventListener("click", function() {
    var currentValue = parseFloat(numberInput.value);
    var step = parseFloat(numberInput.step);
    var newValue = currentValue + step;
    numberInput.value = newValue;
  });

  stepDownButton.addEventListener("click", function() {
    var currentValue = parseFloat(numberInput.value);
    var step = parseFloat(numberInput.step);
    var newValue = currentValue - step;
    numberInput.value = newValue;
  });
</script>