Is there any reason this shouldn't work?

programming flash
1 

if (itemType == "file") {
			trace("Is File");
			mc_File._visible = true;
			trace("mc_File vis: "+mc_File._visible);
		}

It is tracing “Is File” and then “mc_File vis: undefined”;

This code is inside a ContentItem.as class script and it works the first time but not the second?