Error - list index out of range

Hello,

I have commands.sim file as follows
```

len = startLength
while (len <= endLength)
do
  create specified event type of length len.
  len = len + increment
end

DUPL chr2 25160915 25160925
```

When I run, I get an error of list index out of range. Can someone help me out to fix the error in the commands file

Thank you

Priya