I'm a bit suprised because it works well on my computer.
Is your wand correct ? (in the TOC, you must click on the new gdms file, which correspond to the watershed in raster. Then choose the "wand" tool and click on the raster in the map) --> the new vector layer (called "wand" in the TOC) is created.
Then you can execute your script, which is correct (I made a test)