I assume this isn't coincidental with the changes you just deployed.
I believe it's possible for $results to be empty.
$ arc diff --trace ARGV '/home/ubuntu/phacility/arcanist/bin/../scripts/arcanist.php' 'diff' '--trace' LOAD Loaded "phutil" from "/home/ubuntu/phacility/libphutil/src". LOAD Loaded "arcanist" from "/home/ubuntu/phacility/arcanist/src". Config: Reading user configuration file "/home/ubuntu/.arcrc"... Config: Did not find system configuration at "/etc/arcconfig". Working Copy: Reading .arcconfig from "/home/ubuntu/phacility/phabricator/.arcconfig". Working Copy: Path "/home/ubuntu/phacility/phabricator" is part of `git` working copy "/home/ubuntu/phacility/phabricator". Working Copy: Project root is at "/home/ubuntu/phacility/phabricator". Config: Did not find local configuration at "/home/ubuntu/phacility/phabricator/.git/arc/config". Loading phutil library from '/home/ubuntu/phacility/phabricator/src'... >>> [0] <conduit> user.whoami() <bytes = 117> >>> [1] <http> https://secure.phabricator.com/api/user.whoami <<< [1] <http> 416,009 us <<< [0] <conduit> 417,030 us >>> [2] <exec> $ git diff --no-ext-diff --no-textconv --raw 'HEAD' -- >>> [3] <exec> $ git ls-files --others --exclude-standard <<< [3] <exec> 19,987 us <<< [2] <exec> 28,581 us >>> [4] <exec> $ git diff-files --name-only <<< [4] <exec> 14,060 us >>> [5] <event> diff.didCollectChanges <listeners = 0> <<< [5] <event> 161 us >>> [6] <exec> $ git rev-parse --verify HEAD^ <<< [6] <exec> 2,826 us >>> [7] <exec> $ git rev-parse --abbrev-ref --symbolic-full-name '@{upstream}' <<< [7] <exec> 2,393 us >>> [8] <exec> $ git cat-file -t 'master' <<< [8] <exec> 2,415 us >>> [9] <exec> $ git merge-base 'master' HEAD <<< [9] <exec> 2,639 us >>> [10] <exec> $ git rev-parse 'HEAD' <<< [10] <exec> 2,195 us >>> [11] <exec> $ git log --first-parent --format=medium '33fca12816b6a011d5ff44eee96c2fdbeb46c6e0'..'8cd8b6cfef505ef0ea5524b0a176bcc0ba40c83e' <<< [11] <exec> 2,794 us >>> [12] <exec> $ git log '8cd8b6cfef505ef0ea5524b0a176bcc0ba40c83e' --not '33fca12816b6a011d5ff44eee96c2fdbeb46c6e0' --format='%H%x01%T%x01%P%x01%at%x01%an%x01%aE%x01%s%x01%s%n%n%b%x02' -- <<< [12] <exec> 2,758 us >>> [13] <conduit> differential.query() <bytes = 376> >>> [14] <http> https://secure.phabricator.com/api/differential.query <<< [14] <http> 184,387 us <<< [13] <conduit> 184,857 us [2016-07-31 15:22:10] EXCEPTION: (ConduitClientException) ERR-CONDUIT-CORE: You must use withSourcePHIDs() to query edges. at [<phutil>/src/conduit/ConduitFuture.php:58] arcanist(head=master, ref.master=06c641f92c65), phabricator(head=build_search_api, ref.master=33fca12816b6, ref.build_search_api=8cd8b6cfef50), phutil(head=master, ref.master=d0daa2b343e4) #0 ConduitFuture::didReceiveResult(array) called at [<phutil>/src/future/FutureProxy.php:58] #1 FutureProxy::getResult() called at [<phutil>/src/future/FutureProxy.php:35] #2 FutureProxy::resolve() called at [<phutil>/src/conduit/ConduitClient.php:64] #3 ConduitClient::callMethodSynchronous(string, array) called at [<arcanist>/src/repository/api/ArcanistGitAPI.php:1150] #4 ArcanistGitAPI::loadWorkingCopyDifferentialRevisions(ConduitClient, array) called at [<arcanist>/src/workflow/ArcanistDiffWorkflow.php:1494] #5 ArcanistDiffWorkflow::buildCommitMessage() called at [<arcanist>/src/workflow/ArcanistDiffWorkflow.php:469] #6 ArcanistDiffWorkflow::run() called at [<arcanist>/scripts/arcanist.php:394]