I'm getting this exception whilst trying to update a diff:
arc diff --never-apply-patches --trace HEAD~ libphutil loaded from '/usr/src/libphutil/src'. arcanist loaded from '/usr/src/arcanist/src'. Config: Reading user configuration file "/home/josh/.arcrc"... Config: Did not find system configuration at "/etc/arcconfig". Working Copy: Reading .arcconfig from "/home/josh/workspace/REDACTED/.arcconfig". Working Copy: Path "/home/josh/workspace/REDACTED" is part of `git` working copy "/home/josh/workspace/REDACTED". Working Copy: Project root is at "/home/josh/workspace/REDACTED". Config: Did not find local configuration at "/home/josh/workspace/REDACTED/.git/arc/config". Loading phutil library from '/home/josh/workspace/REDACTED/support/flarc/src'... >>> [0] <conduit> user.whoami() <bytes = 117> >>> [1] <http> https://phabricator.redacted.com/api/user.whoami <<< [1] <http> 2,334,289 us <<< [0] <conduit> 2,334,744 us >>> [2] <exec> $ git diff --no-ext-diff --no-textconv --raw 'HEAD' -- >>> [3] <exec> $ git ls-files --others --exclude-standard <<< [2] <exec> 380,399 us <<< [3] <exec> 511,283 us >>> [4] <exec> $ git diff-files --name-only <<< [4] <exec> 140,486 us >>> [5] <event> diff.didCollectChanges <listeners = 0> <<< [5] <event> 1,042 us >>> [6] <exec> $ git rev-parse 'HEAD' <<< [6] <exec> 14,513 us >>> [7] <exec> $ git merge-base 'HEAD~' 'd5c3af448e725f965ebe3fdaa9f0d66e14c55b1c' <<< [7] <exec> 11,858 us >>> [8] <exec> $ git log --first-parent --format=medium 'be63d51a133c96175bdf9480e318de5802e1751c'..'d5c3af448e725f965ebe3fdaa9f0d66e14c55b1c' <<< [8] <exec> 11,702 us >>> [9] <conduit> differential.query() <bytes = 239> >>> [10] <http> https://phabricator.redacted.com/api/differential.query <<< [10] <http> 616,847 us <<< [9] <conduit> 617,524 us >>> [11] <conduit> differential.query() <bytes = 149> >>> [12] <http> https://phabricator.redacted.com/api/differential.query <<< [12] <http> 518,926 us <<< [11] <conduit> 519,429 us >>> [13] <conduit> differential.getcommitmessage() <bytes = 172> >>> [14] <http> https://phabricator.redacted.com/api/differential.getcommitmessage <<< [14] <http> 585,427 us <<< [13] <conduit> 585,659 us >>> [15] <conduit> differential.parsecommitmessage() <bytes = 537> >>> [16] <http> https://phabricator.redacted.com/api/differential.parsecommitmessage <<< [16] <http> 788,808 us <<< [15] <conduit> 789,192 us [2015-12-21 08:03:56] EXCEPTION: (ArcanistDifferentialCommitMessageParserException) Error parsing field "Reviewers": The objects you have listed include objects which do not exist (#). at [<arcanist>/src/differential/ArcanistDifferentialCommitMessage.php:54] arcanist(head=master, ref.master=9a373c88d747), flarc(), phutil(head=master, ref.master=e0dd9c4efda1) #0 ArcanistDifferentialCommitMessage::pullDataFromConduit(ConduitClient) called at [<arcanist>/src/workflow/ArcanistDiffWorkflow.php:1786] #1 ArcanistDiffWorkflow::getCommitMessageFromRevision(string) called at [<arcanist>/src/workflow/ArcanistDiffWorkflow.php:1518] #2 ArcanistDiffWorkflow::buildCommitMessage() called at [<arcanist>/src/workflow/ArcanistDiffWorkflow.php:460] #3 ArcanistDiffWorkflow::run() called at [<arcanist>/scripts/arcanist.php:382]