diff --git a/src/Symfony/Component/Process/Process.php b/src/Symfony/Component/Process/Process.php
index 6caa46b3562cc..1d03297a33b68 100644
--- a/src/Symfony/Component/Process/Process.php
+++ b/src/Symfony/Component/Process/Process.php
@@ -331,7 +331,9 @@ public function start(callable $callback = null/*, array $env = array()*/)
         } else {
             $envPairs = array();
             foreach ($env as $k => $v) {
-                $envPairs[] = $k.'='.$v;
+                if (false !== $v) {
+                    $envPairs[] = $k.'='.$v;
+                }
             }
         }