NI 9215 open circuit voltage

Hello,

I am using NI 9215 for analog input of +10 to -10 V. I usually use three channels as a differential input. However, with nothing connected to the DAQ (all three channels disconnected), one of the channels read an open voltage of around -10 V while others read around 0 V. I was expecting all to be around -10 V. Can anyone suggest what this means and whether this indicate a problem with the module?

Thank you,

Dushyant.