diff --git a/src/applications/differential/controller/DifferentialDiffCreateController.php b/src/applications/differential/controller/DifferentialDiffCreateController.php
--- a/src/applications/differential/controller/DifferentialDiffCreateController.php
+++ b/src/applications/differential/controller/DifferentialDiffCreateController.php
@@ -27,7 +27,13 @@
     $diff = null;
     // This object is just for policy stuff
     $diff_object = DifferentialDiff::initializeNewDiff($viewer);
-    $repository_phid = null;
+
+    if ($revision) {
+      $repository_phid = $revision->getRepositoryPHID();
+    } else {
+      $repository_phid = null;
+    }
+
     $errors = array();
     $e_diff = null;
     $e_file = null;