What version of Review Board are you running?
3.0.18
What version of Review Bot are you running?
1.0.1
What tool(s) is Review Bot running?
custom, but mirrors flake8 and not tied to a specific bot
What steps will reproduce the problem?

make a small change at the top of the file 


What is the expected output? What do you see instead?
internal error in the code review
Please provide any additional information below.
In bot/reviewbot/processing:_is_modified, there's logic to determine the index of a diff line based on the value of the original variable. This does not seem to work when there's a small line change near the top of the file. 
If the original row[line_num_index] value returns a string, swapping the line_num_index to the other value appears to at lease remove the error. But this is hack, not familiar enough with the code base