buildmap.py fails to process prefix_asns.out

[jahway603]

When following the instructions here, it fails when prefix_asns.out is fed to ../buildmap.py.

$ python ../buildmap.py prefix_asns.out 
Traceback (most recent call last):
  File "MY_PATH/asmap/remote_dumps/../buildmap.py", line 147, in <module>
    def PropTree(tree, approx=True) -> (list, Counter, bool):
NameError: name 'Counter' is not defined

Any suggestions on how to fix this?