diff --git a/src/difference/ArcanistDiffVectorTree.php b/src/difference/ArcanistDiffVectorTree.php
index f0a3b72d..8573b939 100644
--- a/src/difference/ArcanistDiffVectorTree.php
+++ b/src/difference/ArcanistDiffVectorTree.php
@@ -1,95 +1,95 @@
 <?php
 
 final class ArcanistDiffVectorTree
   extends Phobject {
 
-  private $vectors;
+  private $vectors = array();
 
   public function addVector(array $vector) {
     $this->vectors[] = $vector;
     return $this;
   }
 
   public function newDisplayList() {
     $root = new ArcanistDiffVectorNode();
 
     foreach ($this->vectors as $vector) {
       $root->addChild($vector, count($vector), 0);
     }
 
     foreach ($root->getChildren() as $child) {
       $this->compressTree($child);
     }
 
     $root->setDisplayDepth(-1);
     foreach ($root->getChildren() as $child) {
       $this->updateDisplayDepth($child);
     }
 
     return $this->getDisplayList($root);
   }
 
   private function compressTree(ArcanistDiffVectorNode $node) {
     $display_node = $node;
 
     $children = $node->getChildren();
     if ($children) {
       $parent = $node->getParentNode();
       if ($parent) {
         $siblings = $parent->getChildren();
         if (count($siblings) === 1) {
           if (!$parent->getValueNode()) {
             $parent_display = $parent->getDisplayNode();
             if ($parent_display) {
               $display_node = $parent_display;
               if ($node->getValueNode()) {
                 $parent->setValueNode($node->getValueNode());
               }
             }
           }
         }
       }
     }
 
     $node->setDisplayNode($display_node);
 
     $display_element = last($node->getVector());
     $display_node->appendDisplayElement($display_element);
 
     foreach ($children as $child) {
       $this->compressTree($child);
     }
   }
 
   private function updateDisplayDepth(ArcanistDiffVectorNode $node) {
     $parent_depth = $node->getParentNode()->getDisplayDepth();
 
     if ($node->getDisplayVector() === null) {
       $display_depth = $parent_depth;
     } else {
       $display_depth = $parent_depth + 1;
     }
 
     $node->setDisplayDepth($display_depth);
 
     foreach ($node->getChildren() as $child) {
       $this->updateDisplayDepth($child);
     }
   }
 
   private function getDisplayList(ArcanistDiffVectorNode $node) {
     $result = array();
 
     foreach ($node->getChildren() as $child) {
       if ($child->getDisplayVector() !== null) {
         $result[] = $child;
       }
       foreach ($this->getDisplayList($child) as $item) {
         $result[] = $item;
       }
     }
 
     return $result;
   }
 
 }