IAC getCVStatus error

When using IAC communication variable. I tried to connect to a getCVstatus block. It is not giving error when I am checking for errors inside program. But when I am checking for by errors outside using right click and check. It is giving unsupported assignment. Error 1090. This is happening in 800xA 5.1 Rev A system control builder. Did anyone face this issue? I have used same in 800xA 6.1 version. There it didn't give any problem.