diff --git a/src/applications/differential/engine/DifferentialRevisionTimelineEngine.php b/src/applications/differential/engine/DifferentialRevisionTimelineEngine.php
--- a/src/applications/differential/engine/DifferentialRevisionTimelineEngine.php
+++ b/src/applications/differential/engine/DifferentialRevisionTimelineEngine.php
@@ -13,7 +13,7 @@
       $view_data = array();
     }
 
-    $left = idx($view_data, 'left');
+    $left = idx($view_data, 'left', 0);
     $right = idx($view_data, 'right');
 
     $diffs = id(new DifferentialDiffQuery())