Build failing after branch for PR deleted

I’m just now starting to work with Drone, so this could be something I’ve not configured properly, but…

I have a build that is properly triggered on a new PR in GitHub Enterprise. The build is successful. When I close the PR and delete the branch, Drone runs a new build that fails with errors like this:

git fetch --no-tags origin +refs/pull/113/merge:
fatal: Couldn’t find remote ref refs/pull/113/merge
retry in 5s

  • git fetch --no-tags origin +refs/pull/113/merge:
    fatal: Couldn’t find remote ref refs/pull/113/merge
    retry in 5s
  • git fetch --no-tags origin +refs/pull/113/merge:
    fatal: Couldn’t find remote ref refs/pull/113/merge
    retry in 5s
  • git fetch --no-tags origin +refs/pull/113/merge:
    fatal: Couldn’t find remote ref refs/pull/113/merge
    retry in 5s
  • git fetch --no-tags origin +refs/pull/113/merge:
    fatal: Couldn’t find remote ref refs/pull/113/merge
    retry in 5s
  • git fetch --no-tags origin +refs/pull/113/merge:
    fatal: Couldn’t find remote ref refs/pull/113/merge
    exit status 128

Is there a way to prevent this?