Error running demo.py

I get the following error when I run the demo.py file:

```
/python-gearbox-master/gearbox/standards/iso.py in __yb(gear)
    909             return 1.6 * log(2.242 * 1 / srh)
    910         else:
--> 911             raise Exception('External gears require: sr/ht > 0.5')
    912 
    913 
Exception: External gears require: sr/ht > 0.5
```