diff --git a/src/applications/differential/parser/DifferentialChangesetParser.php b/src/applications/differential/parser/DifferentialChangesetParser.php
--- a/src/applications/differential/parser/DifferentialChangesetParser.php
+++ b/src/applications/differential/parser/DifferentialChangesetParser.php
@@ -996,9 +996,9 @@
 
         $line = $comment->getLineNumber();
         if ($new_side) {
-          $back_line = $new_backmap[$line];
+          $back_line = idx($new_backmap, $line);
         } else {
-          $back_line = $old_backmap[$line];
+          $back_line = idx($old_backmap, $line);
         }
 
         if ($back_line != $line) {
diff --git a/src/applications/differential/parser/__tests__/DifferentialChangesetParserTestCase.php b/src/applications/differential/parser/__tests__/DifferentialChangesetParserTestCase.php
--- a/src/applications/differential/parser/__tests__/DifferentialChangesetParserTestCase.php
+++ b/src/applications/differential/parser/__tests__/DifferentialChangesetParserTestCase.php
@@ -33,7 +33,7 @@
     }
   }
 
-  public function testDiffBinary() {
+  public function testRenderChangeset() {
     $hunk = id(new DifferentialModernHunk())
       ->setChanges('');
 
@@ -51,11 +51,11 @@
     $parser = id(new DifferentialChangesetParser())
       ->setChangeset($changeset)
       ->setMarkupEngine($engine)
-      //->setMask(array())
+      ->setMask(array())
       ->setUser($viewer)
       ->parseInlineComment($inline);
 
-    $this->assertEqual(null, $parser->renderChangeset());
+    $this->assertEqual('', $parser->renderChangeset());
   }
 
 }