No property with the name '_x'

1

Hello -
I’m using the below code, and when I test my movie, I get the following message in the output panel

Error Scene=Scene 1, layer=Actions, frame=1:Line 10: There is no property with the name ‘_x’.
MessageX._x = 7;
Error Scene=Scene 1, layer=Actions, frame=1:Line 11: There is no property with the name ‘_y’.
MessageX._y = (PrevMessage._y + PrevMessage._height);

Any suggestions?

function attachMessage(n:Number):Void{
 var MessageX:String = ("Message" + n);
 trace (MessageX);
 this.attachMovie("mcMessageHolder", MessageX, this.getNextHighestDepth());
 if (MessageX != "Message0"){
  var PrevMessage:String = ("Message" + n--);
  trace(PrevMessage);
  MessageX._x = 7;
  MessageX._y = (PrevMessage._y + PrevMessage._height);
 }
 else{
  Message0._x = 7;
  Message0._y = 79;
 }
}