diff --git a/src/Symfony/Bridge/PhpUnit/bin/simple-phpunit b/src/Symfony/Bridge/PhpUnit/bin/simple-phpunit
index 49b88ed969a3..7afcbeedc768 100755
--- a/src/Symfony/Bridge/PhpUnit/bin/simple-phpunit
+++ b/src/Symfony/Bridge/PhpUnit/bin/simple-phpunit
@@ -94,8 +94,9 @@ if (!file_exists("$PHPUNIT_DIR/phpunit-$PHPUNIT_VERSION/phpunit") || md5_file(__
     }
     $prevRoot = getenv('COMPOSER_ROOT_VERSION');
     putenv("COMPOSER_ROOT_VERSION=$PHPUNIT_VERSION.99");
+    $q = '\\' === DIRECTORY_SEPARATOR ? '"' : '';
     // --no-suggest is not in the list to keep compat with composer 1.0, which is shipped with Ubuntu 16.04LTS
-    $exit = proc_close(proc_open("$COMPOSER install --no-dev --prefer-dist --no-progress --ansi", array(), $p, getcwd()));
+    $exit = proc_close(proc_open("$q$COMPOSER install --no-dev --prefer-dist --no-progress --ansi$q", array(), $p, getcwd()));
     putenv('COMPOSER_ROOT_VERSION'.(false !== $prevRoot ? '='.$prevRoot : ''));
     if ($exit) {
         exit($exit);