I assume this isn't coincidental with the changes you just deployed.
https://secure.phabricator.com/diffusion/P/browse/master/src/applications/differential/conduit/DifferentialConduitAPIMethod.php;1b192f746a6751e219c30fc45cbf3d05708eec2c$200
I believe it's possible for `$results` to be empty.
```lang=bash
$ 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]
```