symbols = ["MON", "AAPL", "CSRA"]
for symbol in symbols:
qb.AddEquity(symbol)
end_date = datetime.now()
start_date = end_date - timedelta(days=60)
history = qb.History(symbols, start_date, end_date, Resolution.Daily)

I'm running the above code in a notebook. If I try to do any of these, I get an error.
 

history.loc["MON"]
# KeyError: 'the label [MON] is not in the [index]'

history.loc["CSRA"]
# KeyError: 'the label [CSRA] is not in the [index]'

history.loc["AAPL"]
# works fine

Both MON and CSRA appears to be in your available daily data:
https://www.quantconnect.com/data/file/equity/usa/daily/c*/csra.zip/csra.csv
https://www.quantconnect.com/data/file/equity/usa/daily/m*/mon.zip/mon.csv

 

How comes this crashes my notebook?

Thanks!