XMLTextNode.Value not assignable

I have an XML document similar to plists on Mac OS X:

<?xml version="1.0" encoding="UTF-8"?>

I successfully can retrieve the text node which holds the value “ABC”, but I can not change the value. I get an XMLException with the number 0.

Dim node As XmlNode = Document.XQL("dict/key[text()='MyKey']/following-sibling::*[1]").Item(0)
If node.Name = "string" Then
  Dim textNode As XmlTextNode = XmlTextNode(node.FirstChild)
  MsgBox(textNode.Value)   // this works and shows "ABC"
  textNode.Value = "XXX"   // <--- XMLException with number 0
ElseIf node.Name = ...

What am I doing wrong? Do I need to delete the node and replace it (this shouldn’t be necessary as far as I have been able to understand from the Expat website).

Sorry, was my mistake - everything works fine now. Got lost in the depths of ifs and selects and I was raising an error in an inappropriate place.