Fix #106

                      
|Q            |A     |
|---          |---   |
|Bug Fix?     |y     |
|New Feature? |n     |
|BC Breaks?   |Unsure|
|Deprecations?|n     |
|Tests Pass?  |y     |
|Fixed Tickets|#106  |
|License      |MIT   |
|Doc PR       |      |
                      

Fix #106, where the base branch is invalid for a pull request. I'm not sure how will affect github enterprise, as I can't test it