QGIS » QGIS Application

I was trying to clip dem by watersheds.
It was same process as below.

http://docs.qgis.org/2.2/ja/docs/training_manual/processing/iterative.html

But it doesn't work when it iterates.
Without iteration, I could clip my data.
I made sure that polygon(watersheds) and raster(dem) have same CRS.

I think this function is very useful for everyone.
Please give me some idea to fix this problem.

error message from QGIS
AttributeError: 'NoneType' object has no attribute 'error'
Traceback (most recent call last):
File "C:/PROGRA~1/QGIS21~1.1/apps/qgis/./python/plugins\\processing\\gui\\AlgorithmDialog.py", line 238, in accept
if runalgIterating(self.alg, self.iterateParam, self):
File "C:/PROGRA~1/QGIS21~1.1/apps/qgis/./python/plugins\\processing\\gui\\AlgorithmExecutor.py", line 90, in runalgIterating
if runalg(alg):
File "C:/PROGRA~1/QGIS21~1.1/apps/qgis/./python/plugins\\processing\\gui\\AlgorithmExecutor.py", line 52, in runalg
progress.error(e.msg)
AttributeError: 'NoneType' object has no attribute 'error'

Python version: 2.7.5 (default, May 15 2013, 22:44:16) [MSC v.1500 64 bit (AMD64)]
QGIS version: 2.16.1-Nødebo Nødebo, 8545b3b