--- a/js/src/tests/lib/tests.py
+++ b/js/src/tests/lib/tests.py
@@ -77,18 +77,18 @@ class Test(object):
             return [ '-f', 'shell.js' ]
         head, base = os.path.split(path)
         return Test.prefix_command(head) + [ '-f', os.path.join(path, 'shell.js') ]
 
     def get_command(self, js_cmd_prefix):
         dirname, filename = os.path.split(self.path)
         cmd = js_cmd_prefix
         if self.allowXml:
-            cmd += [ '-e', 'options("allow_xml")' ]
-        cmd += Test.prefix_command(dirname)
+            cmd = cmd + [ '-e', 'options("allow_xml")' ]
+        cmd = cmd + Test.prefix_command(dirname)
         if self.debugMode:
             cmd += [ '-d' ]
         # There is a test that requires the path to start with './'.
         cmd += [ '-f', './' + self.path ]
         return cmd
 
     def run(self, js_cmd_prefix, timeout=30.0):
         cmd = self.get_command(js_cmd_prefix)