TypeError: 'ZCylinder' object is not iterable

Hi guys,
I use the python API to build my case. The same method work fine for dimension 2X2, but fail in dimension 3X3. I am so sorry if the question is easy enough. I can not solve it for a few days, anything look fine, but it could not run safely.
The error is :

my python script to define the model below the file(dimension2X2), it works fine:


And my python script to define the model below the file(dimension3X3), it works fail:


Could someone help me out!
Thanks Advance!


Best regards!
Zhuangli