--- a/testing/web-platform/tests/tools/webdriver/webdriver/client.py
+++ b/testing/web-platform/tests/tools/webdriver/webdriver/client.py
@@ -531,17 +531,22 @@ class Session(object):
         else:
             url = "frame"
             body = {"id": frame}
 
         return self.send_session_command("POST", url, body)
 
     @command
     def close(self):
-        return self.send_session_command("DELETE", "window")
+        handles = self.send_session_command("DELETE", "window")
+        if len(handles) == 0:
+            # With no more open top-level browsing contexts, the session is closed.
+            self.session_id = None
+
+        return handles
 
     @property
     @command
     def handles(self):
         return self.send_session_command("GET", "window/handles")
 
     @property
     @command