TypeError: 'NoneType' object is not subscriptable in report_ftk_extents.py

[arhonemus]

When running report_ftk_extents.py the following error occurred and no JSON was created.
Traceback (most recent call last):
File "/mnt/c/Users/foren/Documents/report_ftk_extents.py", line 343, in
main()
File "/mnt/c/Users/foren/Documents/report_ftk_extents.py", line 330, in main
ers = create_er_list(tree)
File "/mnt/c/Users/foren/Documents/report_ftk_extents.py", line 115, in create_er_list
audit_ers(ers)
File "/mnt/c/Users/foren/Documents/report_ftk_extents.py", line 124, in audit_ers
er_number = int(number[1])
TypeError: 'NoneType' object is not subscriptable