My 401(k) at Principal says the return is "N/A". The fine print says "The personalized rate of return for this investment option is not available because it may not be a true reflection of the rate earned for the time period specified on the display. Due to short-term volatility in certain investment markets, we may require some time to pass following a significant transaction before displaying a personalized rate of return."
That translates to "It's down more than 50% and we won't show you the results". Every time the market goes up and the return gets to -49% or better, it shows the number. The minute it goes to -50%, it displays as "N/A".