Running sniffer on CC2531 and Instant Contiki

I don't seem be able to get the sniffer working on CC2531 and Instant Contik(VMWare on Windows)i. For example, when I run sensniff, I see this:

user@instant-contiki:~/Downloads$ sudo python sensniff.py -d /dev/ttyACM1  -D INFO
Started logging
Serial port /dev/ttyACM1 opened
FIFO /tmp/sensniff exists. Using it
Commands:
c: Print current RF Channel
n: Trigger new pcap header before the next frame
h,?: Print this message
[11,26]: Change RF channel
q: Quit
Error reading port: /dev/ttyACM1
The error was: ('device reports readiness to read but returned no data (device disconnected or multiple access on port?)',)
Shutting down
Frame Stats:
           Non-Frame: 0
           Not Piped: 0
               Piped: 0
            Captured: 0

Can you advise what I may have done wrong?

Thanks,

Joe