TypeError: unhashable type

[sh]

This works without a glitch in the command line interpreter:
    segments = {}        # dictionary type
    segment = []
    # populate segment
    segments[segment[0]] = segment[len(segment)-1]

BUT, when I run it from the DOS prompt: "python segments.py", I got error
message:
    TypeError: unhashable type.

Any explanation is appreciated.  Thanks.
- sh