Selenium “Unable to find a matching set of capabilities” despite driver being in /usr/local/bin

I'm trying to follow a tutorial about Selenium, http://selenium-python.readthedocs.io/getting-started.html. I've downloaded the latest version of the geckodriver and copied it to /usr/local/bin. However, when I try

from selenium import webdriver
driver = webdriver.Firefox()
I get the following error message:
Traceback (most recent call last):
  File "/Users/kurtpeek/Documents/Scratch/selenium_getting_started.py", line 4, in 
    driver = webdriver.Firefox()
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/selenium/webdriver/firefox/webdriver.py", line 152, in __init__
    keep_alive=True)
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/selenium/webdriver/remote/webdriver.py", line 98, in __init__
    self.start_session(desired_capabilities, browser_profile)
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/selenium/webdriver/remote/webdriver.py", line 188, in start_session
    response = self.execute(Command.NEW_SESSION, parameters)
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/selenium/webdriver/remote/webdriver.py", line 252, in execute
    self.error_handler.check_response(response)
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/selenium/webdriver/remote/errorhandler.py", line 194, in check_response
    raise exception_class(message, screen, stacktrace)
selenium.common.exceptions.WebDriverException: Message: Unable to find a matching set of capabilities
[Finished in 1.2s with exit code 1]

From https://github.com/SeleniumHQ/selenium/issues/3884, it seems like other users are experiencing similar issues, but the Selenium team is unable to reproduce it. How can I get Selenium working with Firefox? (It does work with chromedriver and a webdriver.Chrome() instance, so I suspect this might be a bug in Selenium).