Error from gis extension of gis:vertex-lists-of function

[Ll Nu]

Hello, 
I want to extract points from line feature from shapefile.
I have search for solutions, it shows that most of people use gis:vertex-lists-of to do this.
However I got an error message below when I try to do that:

Extension exception: not a VectorFeature: org.myworldgis.netlogo.VectorDataset @ca24265
error while observer running GIS:VERTEX-LISTS-OF
  called by procedure DRAW-CITIES
  called by Button 'draw-cities'

Code below:

to draw-cities
  gis:set-drawing-color red
  gis:draw cities-dataset 1
  foreach gis:vertex-lists-of cities-dataset [
    a ->
  ]
end

cities-dataset is a points collection shapefile from Netlogo library ""GIS general examples" model

I don't understand that even I put a points feature in the function, I just got an error. 
Do I misuse the function? How can I corectly use it?  Thank you.