I'm getting the error (in the subject) and I'm not sure where to begin debugging. Here's the full stack trace from $phd log
```
[2016-11-10 07:36:20] EXCEPTION: (Exception) Can't parse an empty diff! at [<arcanist>/src/parser/ArcanistDiffParser.php:191]
arcanist(head=stable, ref.master=9c056c5cc887, ref.stable=e17fe43ca3fe), phabricator(head=stable, ref.master=b441e8b81e31, ref.stable=111c63955155), phutil(head=stable, ref.master=c72eb747e706, ref.stable=40e473e05788)
#0 <#2> ArcanistDiffParser::parseDiff(string) called at [<phabricator>/src/applications/diffusion/herald/HeraldCommitAdapter.php:244]
#1 <#2> HeraldCommitAdapter::loadCommitDiff() called at [<phabricator>/src/applications/diffusion/herald/HeraldCommitAdapter.php:259]
#2 phlog(Exception) called at [<phabricator>/src/applications/diffusion/herald/HeraldCommitAdapter.php:262]
#3 HeraldCommitAdapter::loadDiffContent(string) called at [<phabricator>/src/applications/diffusion/herald/DiffusionCommitDiffContentAddedHeraldField.php:17]
#4 DiffusionCommitDiffContentAddedHeraldField::getHeraldFieldValue(PhabricatorRepositoryCommit) called at [<phabricator>/src/applications/herald/adapter/HeraldAdapter.php:102]
#5 HeraldAdapter::getHeraldField(string) called at [<phabricator>/src/applications/herald/engine/HeraldEngine.php:370]
#6 HeraldEngine::getObjectFieldValue(string) called at [<phabricator>/src/applications/herald/engine/HeraldEngine.php:365]
#7 HeraldEngine::getConditionObjectValue(HeraldCondition, HeraldCommitAdapter) called at [<phabricator>/src/applications/herald/engine/HeraldEngine.php:277]
#8 HeraldEngine::doesRuleMatch(HeraldRule, HeraldCommitAdapter) called at [<phabricator>/src/applications/herald/engine/HeraldEngine.php:91]
#9 HeraldEngine::applyRules(array, HeraldCommitAdapter) called at [<phabricator>/src/applications/herald/engine/HeraldEngine.php:43]
#10 HeraldEngine::loadAndApplyRules(HeraldCommitAdapter) called at [<phabricator>/src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php:3088]
#11 PhabricatorApplicationTransactionEditor::applyHeraldRules(PhabricatorRepositoryCommit, array) called at [<phabricator>/src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php:1008]
#12 PhabricatorApplicationTransactionEditor::applyTransactions(PhabricatorRepositoryCommit, array) called at [<phabricator>/src/applications/repository/worker/PhabricatorRepositoryCommitHeraldWorker.php:102]
#13 PhabricatorRepositoryCommitHeraldWorker::parseCommit(PhabricatorRepository, PhabricatorRepositoryCommit) called at [<phabricator>/src/applications/repository/worker/PhabricatorRepositoryCommitParserWorker.php:51]
#14 PhabricatorRepositoryCommitParserWorker::doWork() called at [<phabricator>/src/infrastructure/daemon/workers/PhabricatorWorker.php:123]
#15 PhabricatorWorker::executeTask() called at [<phabricator>/src/infrastructure/daemon/workers/storage/PhabricatorWorkerActiveTask.php:171]
#16 PhabricatorWorkerActiveTask::executeTask() called at [<phabricator>/src/infrastructure/daemon/workers/PhabricatorTaskmasterDaemon.php:22]
#17 PhabricatorTaskmasterDaemon::run() called at [<phutil>/src/daemon/PhutilDaemon.php:181]
#18 PhutilDaemon::execute() called at [<phutil>/scripts/daemon/exec/exec_daemon.php:127]
```