diff --git a/src/future/Future.php b/src/future/Future.php
--- a/src/future/Future.php
+++ b/src/future/Future.php
@@ -14,6 +14,7 @@
   private $exception;
   private $futureKey;
   private $serviceProfilerCallID;
+  private static $nextKey = 1;
 
   /**
    * Is this future's process complete? Specifically, can this future be
@@ -242,10 +243,8 @@
   }
 
   final public function getFutureKey() {
-    static $next_key = 1;
-
     if ($this->futureKey === null) {
-      $this->futureKey = sprintf('Future/%d', $next_key++);
+      $this->futureKey = sprintf('Future/%d', self::$nextKey++);
     }
 
     return $this->futureKey;
diff --git a/src/future/FutureIterator.php b/src/future/FutureIterator.php
--- a/src/future/FutureIterator.php
+++ b/src/future/FutureIterator.php
@@ -58,12 +58,8 @@
   public function __construct(array $futures) {
     assert_instances_of($futures, 'Future');
 
-    $respect_keys = !phutil_is_natural_list($futures);
-
     foreach ($futures as $map_key => $future) {
-      if ($respect_keys) {
-        $future->setFutureKey($map_key);
-      }
+      $future->setFutureKey($map_key);
       $this->addFuture($future);
     }
   }