error in the __call__ method of class Script

If "client" is a pipeline, and this pipeline is empty, python converts "client" to False when casting in bool because Class BasePipeline has a method __len__. 
"client" is reset to self.registered_client, and the script is not executed in the pipeline as it should.
The correct test is to check if "client" equals None.

see http://stackoverflow.com/questions/100732/why-is-if-not-someobj-better-than-if-someobj-none-in-python for bool conversion (first answer).