set unknown_cost_value in costmap

edit

How can I set "unknown_cost_value" in costmap?

In /map there are unknown value set to -1, but when /map is converted into costmap, unknow value is set to 0, like free space.

I tried to set parameters in costmap_common_params.yaml but it doesn't works

   global_costmap:
   global_frame: /map
   robot_base_frame: /base_footprint
   update_frequency: 4.0
   publish_frequency: 4.0
   static_map: true
   static_layer: {enabled: true, lethal_cost_threshold: 100, track_unknown_space: true, unknown_cost_value: -1}   
   transform_tolerance: 0.5

Thanks