why does period display NaN? what does "NaN" mean?

I have a vi that is supposed to measure 3 period values and 2 voltages using DAQmx.  The vi is attached. 

When I run the vi, I get no errors, but, one of the counters is displaying NaN instead of a period value (in seconds).  Why would this counter not show up?  See the .jpg image of the screen capture, which shows the front panel indicators, after running and stopping the vi.  Notice that period 2 shows "NaN" but, period 1 and 3 display a real value.  The "NaN" displays only in "Period 2" during the execution too. "Period 2" never displays a numeric value.  By the way, what does "NaN" actually mean?

Appreciate your thoughts.

Thanks,

Dave

(using 3 counters in the cDAQ 9174 and NI 9401 module plugged into the cDAQ chassis; periods are measured from encoders)