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
@@ -63,7 +63,11 @@
   }
 
   public function getRequiredHandlePHIDsForEdit() {
-    return mpull($this->getValue(), 'getReviewerPHID');
+    $phids = array();
+    if ($this->getValue()) {
+      $phids = mpull($this->getValue(), 'getReviewerPHID');
+    }
+    return $phids;
   }
 
   public function renderEditControl(array $handles) {