Fails to open build shell after build failure

I have a local workspace open for WebKit, and have modified it such that it fails to build. BuildStream gives me an interactive prompt to decide what to do, and I decide I want a shell:

Build failure on element: core-deps/WebKit.bst

Choose one of the following options:
  (c)ontinue  - Continue queueing jobs as much as possible
  (q)uit      - Exit after all ongoing jobs complete
  (t)erminate - Terminate any ongoing jobs and exit
  (r)etry     - Retry this job
  (l)og       - View the full log file
  (s)hell     - Drop into a shell in the failed build sandbox

Pressing ^C will terminate jobs and exit

Choice: [continue]: s

Problem is it opens a runtime shell, not a build shell. I have no access to the build directory and no ability to do anything useful inside the shell. Surely that's not correct?