diff --git a/tools/wptrunner/wptrunner/browsers/servodriver.py b/tools/wptrunner/wptrunner/browsers/servodriver.py
index 6486a788b8c8fb..d788effc469bc2 100644
--- a/tools/wptrunner/wptrunner/browsers/servodriver.py
+++ b/tools/wptrunner/wptrunner/browsers/servodriver.py
@@ -106,7 +106,7 @@ def start(self, **kwargs):
             self.binary,
             self.binary_args + [
                 "--hard-fail",
-                "--webdriver", str(self.webdriver_port),
+                "--webdriver=%s" % self.webdriver_port,
                 "about:blank",
             ],
             self.debug_info
diff --git a/tools/wptrunner/wptrunner/webdriver_server.py b/tools/wptrunner/wptrunner/webdriver_server.py
index 9ecd009a7d8127..4ec415ce78bb89 100644
--- a/tools/wptrunner/wptrunner/webdriver_server.py
+++ b/tools/wptrunner/wptrunner/webdriver_server.py
@@ -197,7 +197,7 @@ def __init__(self, logger, binary="servo", binary_args=None, host="127.0.0.1",
 
     def make_command(self):
         command = [self.binary,
-                   "--webdriver", str(self.port),
+                   "--webdriver=%s" % self.port,
                    "--hard-fail",
                    "--headless"] + self._args
         if self.binary_args: