Bison: {{ if first }} doesn’t work properly inside {{ bison:item_options }}

I'm trying to use the follow code:

{{ bison:item_options product="{url}" select="no" }}
    <label for="{{ sku }}">
        <input id="{{ sku }}" type="radio" name="options" value="{{ sku }}" {{ if first }}checked{{ endif }}>
        (etc)
    </label>
{{ /bison:item_options }}

However, instead of the first, it checks the last one by default. If I change it to “if last”, then the same: it remains checking the last option by default.

Is this a bug or I'm doing something wrong?