Select-element with multiple-attribute aren't saved

I have a multi-select element in a template I use with Perch Forms. The element shows up in my template as expected (<select multiple="multiple" ...), but the selected values aren't saved in the backend or e-mailed after submitting the form.

Only one value shows up in the e-mail and backend.

This is the markup I use in the template:

<perch:input type="select" id="options" multiple="true" size="10" options="ABS, Airbag, Airconditioning, Alarm, Boardcomputer, Bochtverlichting, Carkit, Climate control, Centrale vergrendeling, Cruise control">

Is there something I'm missing?