diff -u b/core/modules/simpletest/simpletest.module b/core/modules/simpletest/simpletest.module
--- b/core/modules/simpletest/simpletest.module
+++ b/core/modules/simpletest/simpletest.module
@@ -246,10 +246,10 @@
     }, $unescaped_test_classnames);
 
     $filter_string = implode("|", $escaped_test_classnames);
-    $command += array(
+    $command = array_merge($command, array(
       '--filter',
       escapeshellarg($filter_string),
-    );
+    ));
   }
 
   // Need to change directories before running the command so that we can use