Category slug variable is not available when category url's are disabled Just like the title states, the slug variable is not available when a category's url are disabled:

&lt;select name="field-styles" id="field-styles" class="select"&gt;
    &lt;option value=""&gt;Style&lt;/option&gt;
    {% for style in craft.categories.group('productStyles').order('title') %}
    &lt;option value="{{ style.slug }}"&gt;{{ style.title }}&lt;/option&gt;
    {% endfor %}
&lt;/select&gt;


The snippet of code above works when the URL's are enabled for this category group. When disabled, the input value is blank.
That's by design, slugs are used for building the url to link to a specific element, in this case, a category.

Edit: seems I was wrong on the above, see comment. Leaving the rest of the answer as using the ID offers a solution. 

You'll have to use the id to refer to the category as value, like so: 

&lt;select name="field-styles" id="field-styles" class="select"&gt;
    &lt;option value=""&gt;Style&lt;/option&gt;
    {% for style in craft.categories.group('productStyles').order('title') %}
    &lt;option value="{{ style.id }}"&gt;{{ style.title }}&lt;/option&gt;
    {% endfor %}
&lt;/select&gt;