So I've been backtesting momentum factors on long-only unleveraged equities lately but noticed in my latest backtest the portfolio value catastrophically dropped to a negative balance? Why is this happening and what can I do to fix this? I had this issue occur on other strategies I have been experimenting before with but they are all unleveraged long only strategies. I don't usually expect investing long in stocks without leverage would suddenly bring your portfolio to a negative balance and get margin called out of the blue in a single day………..

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