--- a/testing/web-platform/tests/tools/webdriver/webdriver/client.py
+++ b/testing/web-platform/tests/tools/webdriver/webdriver/client.py
@@ -11,17 +11,16 @@ def command(func):
     def inner(self, *args, **kwargs):
         if hasattr(self, "session"):
             session = self.session
         else:
             session = self
 
         if session.session_id is None:
             session.start()
-        assert session.session_id is not None
 
         return func(self, *args, **kwargs)
 
     inner.__name__ = func.__name__
     inner.__doc__ = func.__doc__
 
     return inner
 
@@ -660,29 +659,29 @@ class Element(object):
 
         :param id: Web element UUID which must be unique across
             all browsing contexts.
         :param session: Current ``webdriver.Session``.
         """
         self.id = id
         self.session = session
 
-        assert id not in self.session._element_cache
+        if id in self.session._element_cache:
+            raise ValueError("Element already in cache: %s" % id)
         self.session._element_cache[self.id] = self
 
     def __repr__(self):
         return "<%s %s>" % (self.__class__.__name__, self.id)
 
     def __eq__(self, other):
         return (isinstance(other, Element) and self.id == other.id and
                 self.session == other.session)
 
     @classmethod
     def from_json(cls, json, session):
-        assert Element.identifier in json
         uuid = json[Element.identifier]
         if uuid in session._element_cache:
             return session._element_cache[uuid]
         return cls(uuid, session)
 
     def send_element_command(self, method, uri, body=None):
         url = "element/%s/%s" % (self.id, uri)
         return self.session.send_session_command(method, url, body)