[FLOW3-general] unset properties on edit form

[Heiko Kromm | Paints]

Hi list, 

is it possible to unset a property on the edit form? 

I've an object "employee" which has a property "supervisor", which is of type "employee". 

In my edit form I use the select-ViewHelper to select a supervisor. 
<f:form.select property="supervisor" id="supervisor" options="{employees}" optionLabelField="name" /> 
This works. 

Now I wan't to have the possibility to unselect an already selected supervisor. 
I tried to extend the options of the select-ViewHelper with an empty value. But if I submit the empty value, I get an 
#1297759968: Exception while property mapping at property path "supervisor":Object with identity "" not found 
Exception. 

Is there a way to handle this with the mappers, or do you have some other suggestions for me? 

Thanks 
Heiko