Scripts don't work anymore

Hi,

I have scripts that used to work, but don't work anymore. Most start with the following:

function xyz()

{

var doc = app.ActiveDoc;
if(doc.ObjectValid()==false)
{
alert("No active document. Cannot continue");
return;
}

...

}

When I run them now, the program stops at the first or second line, with the error "undefined is not an object". 

Why does that happen? I've installed the latest FrameMaker version AFAIK (15.0.5.838).

Thanks in advance for your help!

Aviva