Incident Template - Field & Form Rules

I am trying to set some field & form rules to the default incident template.
I have attached a picture on the setting I am trying to get to work.

I have set the following rule:

Rule execution: On create / edit
Event: On field change - Category

Conditions
Site - contains - 24- (all sites are imported from AD, and sites that starts with 24- is Stockholm in this case)
and
Category - is - IT Services

Actions
Set field - Group - IT Stockholm (I dont get a drop down menu here...)

For some reason this does not work for me. And I think that it is strange that I dont get a drop down menu when I choose which group I want to set field for.
Am I doing something wrong? Is this a bug?

Thanks,
Patrick