diff --git a/src/applications/differential/customfield/DifferentialReviewersField.php b/src/applications/differential/customfield/DifferentialReviewersField.php
--- a/src/applications/differential/customfield/DifferentialReviewersField.php
+++ b/src/applications/differential/customfield/DifferentialReviewersField.php
@@ -187,7 +187,7 @@
     $config_self_accept_key = 'differential.allow-self-accept';
     $allow_self_accept = PhabricatorEnv::getEnvConfig($config_self_accept_key);
 
-    foreach ($value as $phid) {
+    foreach ((array) $value as $phid) {
       if (($phid == $author_phid) && !$allow_self_accept) {
         throw new DifferentialFieldValidationException(
           pht('The author of a revision can not be a reviewer.'));