Error for screenshot_number variable when trying to save a screenshot

gistfile1.txt

<Step: "And I select the "2.0" speed on video "B"">
Traceback (most recent call last):
  File "/edx/app/edxapp/venvs/edxapp/src/lettuce/lettuce/core.py", line 144, in __call__
    ret = self.function(self.step, *args, **kw)
  File "/edx/app/edxapp/edx-platform/lms/djangoapps/courseware/features/video.py", line 336, in change_video_speed_on_video
    change_video_speed(speed)
  File "/edx/app/edxapp/edx-platform/lms/djangoapps/courseware/features/video.py", line 207, in inner
    world.browser.driver.save_screenshot("{0:03d}".format(screenshot_number) + "__" + func.func_name + "__BEFORE.png")
UnboundLocalError: local variable 'screenshot_number' referenced before assignment