我的select input计算器不工作，我如何找出原因？

<select id="operator">
  <option value="+">+</option>
  <option value="-">-</option>
  <option value="*">*</option>
  <option value="/">/</option>
</select>
<input type="number" id="num1">
<input type="number" id="num2">
<button onclick="calculate()">Calculate</button>
<div id="result"></div>

function calculate() {
  const operator = document.getElementById('operator').value;
  const num1 = parseFloat(document.getElementById('num1').value);
  const num2 = parseFloat(document.getElementById('num2').value);
  let result;

  switch (operator) {
    case '+':
      result = num1 + num2;
      break;
    case '-':
      result = num1 - num2;
      break;
    case '*':
      result = num1 * num2;
      break;
    case '/':
      if (num2 === 0) {
        result = 'Error: Division by zero';
      } else {
        result = num1 / num2;
      }
      break;
    default:
      result = 'Invalid operator';
  }

  document.getElementById('result').innerText = result;
}