--- a/testing/marionette/client/marionette/runtests.py
+++ b/testing/marionette/client/marionette/runtests.py
@@ -207,22 +207,22 @@ class MarionetteTestRunner(object):
         self.marionette = None
         self.logcat_dir = logcat_dir
         self.perfrequest = None
         self.xml_output = xml_output
         self.repeat = repeat
         self.perf = perf
         self.perfserv = perfserv
         self.gecko_path = gecko_path
-        self.testvars = None
+        self.testvars = {}
         self.tree = tree
         self.load_early = load_early
         self.device = device
 
-        if testvars is not None:
+        if testvars:
             if not os.path.exists(testvars):
                 raise Exception('--testvars file does not exist')
 
             import json
             with open(testvars) as f:
                 self.testvars = json.loads(f.read())
 
         # set up test handlers
@@ -236,16 +236,17 @@ class MarionetteTestRunner(object):
             self.logger.setLevel(logging.INFO)
             self.logger.addHandler(logging.StreamHandler())
 
         if self.logcat_dir:
             if not os.access(self.logcat_dir, os.F_OK):
                 os.mkdir(self.logcat_dir)
 
         # for XML output
+        self.testvars['xml_output'] = self.xml_output
         self.results = []
 
     def reset_test_stats(self):
         self.passed = 0
         self.failed = 0
         self.todo = 0
         self.failures = []
         self.perfrequest = None