Switched outlet has open neutral when switch is off
Some dopehead put the switch in the neutral wire, which is forbidden by Code as it's a shock hazard.  You'll have to turn power off to the circuit, and then pull both the receptacle and the switch out.  If you see a white wire going to either the brass screws on the receptacle, or any screws on the switch, that's your problem, unless you have a switch loop; in that case, look for a black-flagged wire (or a black wire) wire-nutted to a white wire.  (Someone also could have left  the black paint or tape flags off the switch loop's white wire, and then some dunderhead later on screwed it up without realizing what he was doing...)