FIT dynamic UnitLabels

Hi
I tried to make the unit Labels in FIT dynamically, depending on the user settings. like kmh or knots.
Where speedUnit is the variable:
maxSpeedFitF = DataField.createField(
"Max Speed",
4,
Fit.DATA_TYPE_FLOAT,
{:mesgType=>Fit.MESG_TYPE_SESSION, :units=>speedUnit}

Resources.xml :
<fitField id="4" displayInActivitySummary="true" sortOrder = "4" precision="1"
dataLabel="@Strings.max_speed_label"
unitLabel="@Strings.kmh_units"
/>

But it takes always the string out of unitLabel="@Strings.kmh_units"
and I HAVE to set a unitLabel in resources, otherwise I get a compilation error.
How can I achive this?
Thanks
Seli17