Handle KeyError when retrieving SessionIndex

[rectalogic]

This was broken in commit b69e925
Fixes #826

All Submissions:

• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Have you added an explanation of what problem you are trying to solve with this PR?
• Have you added information on what your changes do and why you chose this as your solution?
• Have you written new tests for your changes?
• Does your submission pass tests?
• This project follows PEP8 style guide. Have you run your code against the 'flake8' linter?

[c00kiemon5ter]

Thanks for the report, the fix and the test @rectalogic

I will prepare a new release with this fix soon.