problem adding attribute using XDocument

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C#

XML

I am checking XML for attribute ProductCount under ProductDetails node, and if the attribute is not present add the attribute with a default value under this node.

I am able to check if the attribute exists or not but I am not able to add it, though it does not give me any error but does not even add the attribiute.

here is my code:

XDocument XMLDoc = XDocument.Load(fileName);

foreach (var detail in XMLDoc.Descendants(_ns + "ProductDetails"))
{
   if (detail.Attribute("ProductCount") == null)
   {
       detail.SetAttributeValue("ProductCount", "1");
   }
}

<productdetails hadsuspension="false" iscomplimentary="false" hasfamily1="false" hasfamily2="false" isinsured="false" hasjoint="false" />

_ns has my namespace

I am not able to figure out what I am doing wrong, why is it not adding ProductCount attribute if it does not exist.