Unable to retrieve treeID after segmentation

I encountered a problem that treeID feature can not be retrieved after implementing Dalponte2016() chm-based segmentation method. The segmentation results can correctly show in multiple colors by using

plot(las_res, bg = "white", size = 2, color = "treeID")

but these treeIDs are actually not available (NA), and I cannot export the treeIDs in both las/laz and .csv formats.