exportAsFDF not trusted

I have made a Pdf-document with a form.

I want to export as FDF - on Acrobat READER.

So I created a .js file, and placed in javascript folder on the computer running the reader.

File -->

var Trusted_handleIndexNo = app.trustedFunction(handleIndexNo);

function handleIndexNo()
{
app.beginPriv();
if (this.getField("indexno").value == "") {
this.importAnFDF("/c/FDFdata/noSerie/indexno_data.fdf");

var v = +this.getField("indexno").value;
this.getField("indexno").value = util.printf("%05d", (v+1));

this.exportAsFDF({cPath: "/c/FDFdata/noSerie/indexno_data.fdf", aFields:["indexno"]});
}
app.endPriv();
}

--> end file

From button in the document I call 

Trusted_handleIndexNo();

But it fails in the Reader-app, but running on Acrobat Pro DC.

Fail: RangeError: Invalid argument value. Doc.exportAsFDF

Can anyone point me in the right direction, please?