为什么在react js下拉列表中不触发onchange?

为什么在react js下拉列表中不触发onchange?我已经绑定了我的函数 context this . 这是我的密码 https://codesandbox.io/s/m2qjz6k48

onDropDownSelected = event => {
    let index = event.nativeEvent.target.selectedIndex;
    //event.nativeEvent.target[index].text;
    if (index !== 0) {
      let fields = this.state.fields;
      fields["selectedRole"] = this.state.roles[index];
      this.setState({ fields });
    }
  };

<Input
              type="select"
              name="select"
              onChange={this.onDropDownSelected.bind(this)}
              className="block__login_formGroup_input"
              //  value={this.state.fields["selectedRole"]}
              id="exampleSelect"
            >
              <option>Select Role</option>
              {this.createRolesDropDown()}
            </Input>