OpenBCI_Python port number error

[parasmemon]

I am using OpenBCI Cyton board. when I use this code:
from pyOpenBCI import OpenBCICyton

def print_raw(sample):
print(sample.channels_data)
board = OpenBCICyton(port=None, daisy=False)
board.start_stream(print_raw)

I get this error: raise OSError('Cannot find OpenBCI port.')
OSError: Cannot find OpenBCI port.