diff --git a/resources/sql/patches/20130523.maniphest_owners.sql b/resources/sql/patches/20130523.maniphest_owners.sql
new file mode 100644
index 0000000000..d5209e2b35
--- /dev/null
+++ b/resources/sql/patches/20130523.maniphest_owners.sql
@@ -0,0 +1 @@
+update {$NAMESPACE}_maniphest.maniphest_task set ownerPHID = NULL WHERE ownerPHID = '';
diff --git a/src/applications/maniphest/storage/ManiphestTask.php b/src/applications/maniphest/storage/ManiphestTask.php
index c20172b268..4c3d83ec2b 100644
--- a/src/applications/maniphest/storage/ManiphestTask.php
+++ b/src/applications/maniphest/storage/ManiphestTask.php
@@ -90,7 +90,7 @@ final class ManiphestTask extends ManiphestDAO
   }
 
   public function setOwnerPHID($phid) {
-    $this->ownerPHID = $phid;
+    $this->ownerPHID = nonempty($phid, null);
     $this->subscribersNeedUpdate = true;
     return $this;
   }
diff --git a/src/infrastructure/storage/patch/PhabricatorBuiltinPatchList.php b/src/infrastructure/storage/patch/PhabricatorBuiltinPatchList.php
index b6d5a23bcd..a67d63211a 100644
--- a/src/infrastructure/storage/patch/PhabricatorBuiltinPatchList.php
+++ b/src/infrastructure/storage/patch/PhabricatorBuiltinPatchList.php
@@ -1310,6 +1310,10 @@ final class PhabricatorBuiltinPatchList extends PhabricatorSQLPatchList {
         'type' => 'sql',
         'name' => $this->getPatchPath('20130521.dropconphimages.sql'),
       ),
+      '20130523.maniphest_owners.sql' => array(
+        'type' => 'sql',
+        'name' => $this->getPatchPath('20130523.maniphest_owners.sql'),
+      ),
 
     );
   }