Historical Bar Data returns None

I switched to using V2 of the market data API and am trying to get the latest 100 Minute bars for a list of symbols. It worked yesterday no problem, but when I ran the same code today, I get the following response.

{'bars': None, 'symbol': 'WM', 'next_page_token': None}

Here is my code:

url = 'https://data.alpaca.markets/v2/stocks/{}/bars?&limit={}&timeframe={}'.format(symbol, limit, timeframe)
response = requests.get(url, headers=config.HEADERS).json()

Any help is greatly appreciated.

According to documentation both start and end query parameters are mandatory.