Error: Bad parameter while hiding/showing template;

Hi,

I have a form  of 4 pages, all 4 have been turned into templates. I have dropdown on MainPage from where user can select Month, according to which corresponding template should show/hide.... ex.. if January is selected then Jan20 template should be visible rest hidden, if February is selected then February should be visible rest hidden.....  Below is the script i have used,, everything is working fine... except everytime is changed the selection it throws error "Bad parameter"

Attached is the link of the file: https://www.dropbox.com/s/5ht33asjvy5r0to/2020.pdf?dl=0

Here is the javascript

var a = this.getTemplate("Main_Page");
var b = this.getTemplate("Jan20");
var c = this.getTemplate("Feb20");
var d = this.getTemplate("Mar20");


var resp = this.getField("Monat").value;
switch (resp) {

case "January 2020":
a.hidden = true;
b.hidden = false;
c.hidden = true;
d.hidden = true;
break;

case "February 2020":
a.hidden = true;
b.hidden = true;
c.hidden = false;
d.hidden = true;
break;

case "March 2020":
a.hidden = true;
b.hidden = true;
c.hidden = true;
d.hidden = false;
break;

case "-Select-":
a.hidden = false;
b.hidden = true;
c.hidden = true;
d.hidden = true;
break;
}

Thanks