Creating pull requests sometimes fail without error message

Sometimes attempting to create a pull request from the web clients results in an error, but the client makes no indication of that fact:

POST to: https://bitbucket.org/hireanesquire/hae/pull-request/new

{"errors":

{"source": ["No such choice 1070-strip-cc-and-bank-fe (117207674233)"]}

, "error": "An error occurred saving this pull request. Please check the required fields and try again"}