Error: must set key for <rc-animate> children

<FormItem {...formItemLayout} label={item.label} key={index}>
  {
    getFieldDecorator(item.paramName, {
      initialValue: [],
    })(
      <Select
        size={config.size}
        showSearch
        mode="multiple"

        optionFilterProp="children"
      >
        {
          item.options.map(option => {
            return (
              <Select.Option key={option.value}
                             value={Util.numToString(option.value)}
              >
                {option.text}
              </Select.Option>

            )
          })
        }
      </Select>
    )
  }
</FormItem>