Local Driveable Areas have "nan" value for z coordinate

[quan-dao]

I am using the function find_local_driveable_areas of class ArgoverseMap to find driveable area around a location on the map. As I check the result, I find there are some driveable areas have nan value for z coordinate. For example, with log_id = c6911883-1843-3727-8eaa-41dc8cda8993 and

timestamp_idx = 100
xcenter, ycenter, _ = argo_data.get_pose(timestamp_idx).translation
xmin = xcenter - 80  
xmax = xcenter + 80  
ymin = ycenter - 80  
ymax = ycenter + 80  
local_das = argo_map.find_local_driveable_areas([xmin, xmax, ymin, ymax], 'PIT')  

The value of local_das[0] is

[[ 679. -201.   nan]
 [ 680. -200.   nan]
 [ 680. -199.   nan]
 ...
 [ 701. -198.   nan]
 [ 699. -200.   nan]
 [ 699. -201.   nan]]

Do these nan values mean this driveable area is not valid?