diff --git a/src/applications/conpherence/editor/ConpherenceEditor.php b/src/applications/conpherence/editor/ConpherenceEditor.php
--- a/src/applications/conpherence/editor/ConpherenceEditor.php
+++ b/src/applications/conpherence/editor/ConpherenceEditor.php
@@ -490,7 +490,7 @@
         foreach ($xactions as $xaction) {
           $phids = $this->getPHIDTransactionNewValue(
             $xaction,
-            $object->getParticipantPHIDs());
+            nonempty($object->getParticipantPHIDs(), array()));
 
           if (!$phids) {
             continue;
diff --git a/src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php b/src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php
--- a/src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php
+++ b/src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php
@@ -1413,7 +1413,7 @@
     PhabricatorApplicationTransaction $xaction,
     $old = null) {
 
-    if ($old) {
+    if ($old !== null) {
       $old = array_fuse($old);
     } else {
       $old = array_fuse($xaction->getOldValue());