How to eliminate output limit of agilent 4156c?

 Hello,

I use agilent 4156c to measure inverter voltage transfer characteristic.

The input(Vin, Vdd, Vss) was measured by SMU mode and output voltage was measured by VMU mode.

However when the output voltage exceeds 23.5V it didn't show the result over 23.5V.

I'm in trouble with this output limit. How can I solve this limit? 

Any help and reply would be really appreciated.

Thank you.