Usage of CMS:FormCategory

Hi,

I created a alternative form for one of my document types and want to group three fields together, so I used a CMS:FormCategory control. Unfortunately the included child controls don't show up:

<cms:FormCategory runat="server" ID="grpContact" CategoryTitle="Contact" DefaultFieldLayout="ThreeColumns">
    <cms:FormField runat="server" ID="fPhone" Field="Phone"/>
    <cms:FormField runat="server" ID="fMobile" Field="Mobile" />
    <cms:FormField runat="server" ID="fEmail" Field="Email" />
</cms:FormCategory>

Shouldn't it work that way?

If I add the control without children (<cms:FormCategory... />), it shows the category header, but of course no fields inside...