diff --git a/src/applications/differential/customfield/DifferentialReviewersField.php b/src/applications/differential/customfield/DifferentialReviewersField.php
index f77f8ab749..41a4cf8f26 100644
--- a/src/applications/differential/customfield/DifferentialReviewersField.php
+++ b/src/applications/differential/customfield/DifferentialReviewersField.php
@@ -1,327 +1,331 @@
 <?php
 
 final class DifferentialReviewersField
   extends DifferentialCoreCustomField {
 
   public function getFieldKey() {
     return 'differential:reviewers';
   }
 
   public function getFieldKeyForConduit() {
     return 'reviewerPHIDs';
   }
 
   public function getFieldName() {
     return pht('Reviewers');
   }
 
   public function getFieldDescription() {
     return pht('Manage reviewers.');
   }
 
   protected function readValueFromRevision(
     DifferentialRevision $revision) {
     return $revision->getReviewerStatus();
   }
 
   public function getNewValueForApplicationTransactions() {
     $specs = array();
     foreach ($this->getValue() as $reviewer) {
       $specs[$reviewer->getReviewerPHID()] = array(
         'data' => $reviewer->getEdgeData(),
       );
     }
 
     return array('=' => $specs);
   }
 
   public function readValueFromRequest(AphrontRequest $request) {
     $datasource = id(new DifferentialBlockingReviewerDatasource())
       ->setViewer($request->getViewer());
 
     $new_phids = $request->getArr($this->getFieldKey());
     $new_phids = $datasource->evaluateTokens($new_phids);
 
     $reviewers = array();
     foreach ($new_phids as $spec) {
       if (!is_array($spec)) {
         $reviewers[$spec] = DifferentialReviewerStatus::STATUS_ADDED;
       } else {
         $reviewers[$spec['phid']] = $spec['type'];
       }
     }
 
     $this->updateReviewers($this->getValue(), $reviewers);
   }
 
   private function updateReviewers(array $old_reviewers, array $new_map) {
     // Compute a new set of reviewer objects. We're going to respect the new
     // reviewer order, add or remove any new or missing reviewers, and respect
     // any blocking or unblocking changes. For reviewers who were there before
     // and are still there, we're going to keep the old value because it
     // may be something like "Accept", "Reject", etc.
 
     $old_map = mpull($old_reviewers, 'getStatus', 'getReviewerPHID');
     $status_blocking = DifferentialReviewerStatus::STATUS_BLOCKING;
 
     $new_reviewers = array();
     foreach ($new_map as $phid => $new) {
       $old = idx($old_map, $phid);
 
       // If we have an old status and this didn't make the reviewer blocking
       // or nonblocking, just retain the old status. This makes sure we don't
       // throw away rejects, accepts, etc.
       if ($old) {
         $is_block = ($old !== $status_blocking && $new === $status_blocking);
         $is_unblock = ($old === $status_blocking && $new !== $status_blocking);
         if (!$is_block && !$is_unblock) {
           $new_reviewers[$phid] = $old;
           continue;
         }
       }
 
       $new_reviewers[$phid] = $new;
     }
 
     foreach ($new_reviewers as $phid => $status) {
       $new_reviewers[$phid] = new DifferentialReviewer(
         $phid,
         array(
           'status' => $status,
         ));
     }
 
     $this->setValue($new_reviewers);
   }
 
   public function renderEditControl(array $handles) {
     $status_blocking = DifferentialReviewerStatus::STATUS_BLOCKING;
 
     $value = array();
     foreach ($this->getValue() as $reviewer) {
       $phid = $reviewer->getReviewerPHID();
       if ($reviewer->getStatus() == $status_blocking) {
         $value[] = 'blocking('.$phid.')';
       } else {
         $value[] = $phid;
       }
     }
 
     return id(new AphrontFormTokenizerControl())
       ->setUser($this->getViewer())
       ->setName($this->getFieldKey())
       ->setDatasource(new DifferentialReviewerDatasource())
       ->setValue($value)
       ->setError($this->getFieldError())
       ->setLabel($this->getFieldName());
   }
 
   public function getApplicationTransactionType() {
     return PhabricatorTransactions::TYPE_EDGE;
   }
 
   public function getApplicationTransactionMetadata() {
     return array(
       'edge:type' => DifferentialRevisionHasReviewerEdgeType::EDGECONST,
     );
   }
 
   public function shouldAppearInPropertyView() {
     return true;
   }
 
   public function renderPropertyViewLabel() {
     return $this->getFieldName();
   }
 
   public function getRequiredHandlePHIDsForPropertyView() {
     return mpull($this->getUserReviewers(), 'getReviewerPHID');
   }
 
   public function renderPropertyViewValue(array $handles) {
     $reviewers = $this->getUserReviewers();
     if (!$reviewers) {
       return phutil_tag('em', array(), pht('None'));
     }
 
     $view = id(new DifferentialReviewersView())
       ->setUser($this->getViewer())
       ->setReviewers($reviewers)
       ->setHandles($handles);
 
     // TODO: Active diff stuff.
 
     return $view;
   }
 
   private function getUserReviewers() {
     $reviewers = array();
     foreach ($this->getObject()->getReviewerStatus() as $reviewer) {
       if ($reviewer->isUser()) {
         $reviewers[] = $reviewer;
       }
     }
     return $reviewers;
   }
 
   public function shouldAppearInCommitMessage() {
     return true;
   }
 
   public function shouldAppearInCommitMessageTemplate() {
     return true;
   }
 
   public function getCommitMessageLabels() {
     return array(
       'Reviewer',
       'Reviewers',
     );
   }
 
   public function parseValueFromCommitMessage($value) {
     $results = $this->parseObjectList(
       $value,
       array(
         PhabricatorPeopleUserPHIDType::TYPECONST,
         PhabricatorProjectProjectPHIDType::TYPECONST,
         PhabricatorOwnersPackagePHIDType::TYPECONST,
       ),
       false,
       array('!'));
 
     return $this->flattenReviewers($results);
   }
 
   public function getRequiredHandlePHIDsForCommitMessage() {
     return mpull($this->getValue(), 'getReviewerPHID');
   }
 
   public function readValueFromCommitMessage($value) {
     $value = $this->inflateReviewers($value);
 
     $reviewers = array();
     foreach ($value as $spec) {
       $phid = $spec['phid'];
 
       $is_blocking = isset($spec['suffixes']['!']);
       if ($is_blocking) {
         $status = DifferentialReviewerStatus::STATUS_BLOCKING;
       } else {
         $status = DifferentialReviewerStatus::STATUS_ADDED;
       }
 
       $reviewers[$phid] = $status;
     }
 
     $this->updateReviewers(
       $this->getObject()->getReviewerStatus(),
       $reviewers);
 
     return $this;
   }
 
   public function renderCommitMessageValue(array $handles) {
     $suffixes = array();
 
     $status_blocking = DifferentialReviewerStatus::STATUS_BLOCKING;
 
     foreach ($this->getValue() as $reviewer) {
       if ($reviewer->getStatus() == $status_blocking) {
         $phid = $reviewer->getReviewerPHID();
         $suffixes[$phid] = '!';
       }
     }
 
     return $this->renderObjectList($handles, $suffixes);
   }
 
   public function validateCommitMessageValue($value) {
+    if (!$value) {
+      return;
+    }
+
     $author_phid = $this->getObject()->getAuthorPHID();
 
     $config_self_accept_key = 'differential.allow-self-accept';
     $allow_self_accept = PhabricatorEnv::getEnvConfig($config_self_accept_key);
 
     $value = $this->inflateReviewers($value);
     foreach ($value as $spec) {
       $phid = $spec['phid'];
 
       if (($phid == $author_phid) && !$allow_self_accept) {
         throw new DifferentialFieldValidationException(
           pht('The author of a revision can not be a reviewer.'));
       }
     }
   }
 
   public function getRequiredHandlePHIDsForRevisionHeaderWarnings() {
     return mpull($this->getValue(), 'getReviewerPHID');
   }
 
   public function getWarningsForRevisionHeader(array $handles) {
     $revision = $this->getObject();
 
     $status_needs_review = ArcanistDifferentialRevisionStatus::NEEDS_REVIEW;
     if ($revision->getStatus() != $status_needs_review) {
       return array();
     }
 
     foreach ($this->getValue() as $reviewer) {
       if (!$handles[$reviewer->getReviewerPHID()]->isDisabled()) {
         return array();
       }
     }
 
     $warnings = array();
     if ($this->getValue()) {
       $warnings[] = pht(
         'This revision needs review, but all specified reviewers are '.
         'disabled or inactive.');
     } else {
       $warnings[] = pht(
         'This revision needs review, but there are no reviewers specified.');
     }
 
     return $warnings;
   }
 
   public function getProTips() {
     return array(
       pht(
         'You can mark a reviewer as blocking by adding an exclamation '.
         'mark ("!") after their name.'),
     );
   }
 
   private function flattenReviewers(array $values) {
     // NOTE: For now, `arc` relies on this field returning only scalars, so we
     // need to reduce the results into scalars. See T10981.
     $result = array();
 
     foreach ($values as $value) {
       $result[] = $value['phid'].implode('', array_keys($value['suffixes']));
     }
 
     return $result;
   }
 
   private function inflateReviewers(array $values) {
     $result = array();
 
     foreach ($values as $value) {
       if (substr($value, -1) == '!') {
         $value = substr($value, 0, -1);
         $suffixes = array('!' => '!');
       } else {
         $suffixes = array();
       }
 
       $result[] = array(
         'phid' => $value,
         'suffixes' => $suffixes,
       );
     }
 
     return $result;
   }
 
 }