selenium能够定位到元素，但是操作元素时候报 Element is no longer valid
from selenium import webdriver
import time
browser = webdriver.Ie()
browser.maximize_window()
browser.get('URL')
elem = browser.find_element_by_css_selector("#username")
elem.click()___
网上给的方法已经都试过了，什么等待，refresh页面等等，都不可以，有没有大神帮忙解决一下呢，错误信息如下
elem.click()
File "C:\Python27\lib\site-packages\selenium-2.52.0-py2.7.egg\selenium\webdriver\remote\webelement.py", line 75, in click
self._execute(Command.CLICK_ELEMENT)
File "C:\Python27\lib\site-packages\selenium-2.52.0-py2.7.egg\selenium\webdriver\remote\webelement.py", line 469, in _execute
return self._parent.execute(command, params)
File "C:\Python27\lib\site-packages\selenium-2.52.0-py2.7.egg\selenium\webdriver\remote\webdriver.py", line 201, in execute
self.error_handler.check_response(response)
File "C:\Python27\lib\site-packages\selenium-2.52.0-py2.7.egg\selenium\webdriver\remote\errorhandler.py", line 194, in check_response
raise exception_class(message, screen, stacktrace)
selenium.common.exceptions.StaleElementReferenceException: Message: Element is no longer valid
