Creating Bookmark with Javascript works fine in Acrobat Pro but not in Acrobat Reader

Hello,

I added a simple JS function in Acrobat Pro DC, that creates a bookmark when a button is clicked. It works fine in Acrobat Pro, but when my client opened it in Acrobat Reader it didn't.

I also added some debugging alerts before and after the bookmark function to check whether the function is called in Reader and to exclude that there is an error in the line itself. Both were fired.

Here is the code:

function addBookmark(label) {
  var index = bookmarkRoot.children ? bookmarkRoot.children.length : 0;
  //bookmarkRoot.createChild(label, "this.pageNum = " + (this.pageNum), index);
  alert("bookmarkRoot'"+this.bookmarkRoot.children[0]+"'");       
  this.bookmarkRoot.createChild("Bookmark label", "this.pageNum = 4");     
  this.bookmarkRoot.createChild("Bookmark label2", '"this.pageNum=4"');      
  alert("finished"); 
}

Any ideas why the bookmarks aren't created and how can I proceed with debugging it?

Thank you in advance,

Peter