diff --git a/src/unit/engine/ArcanistUnitTestEngine.php b/src/unit/engine/ArcanistUnitTestEngine.php
index ad66bb90..e105f216 100644
--- a/src/unit/engine/ArcanistUnitTestEngine.php
+++ b/src/unit/engine/ArcanistUnitTestEngine.php
@@ -6,7 +6,7 @@
 abstract class ArcanistUnitTestEngine extends Phobject {
 
   private $workingCopy;
-  private $paths;
+  private $paths = array();
   private $arguments = array();
   private $enableAsyncTests;
   private $enableCoverage;