Select an attribute with Xpath in a XML with namespace using Powershell

I have a .Net config file with a NLog node inside:

<?xml version="1.0"?>
<configuration>
  <appSettings>
  </appSettings>
  <nlog xmlns="http://ift.tt/JmPwGs"
    xmlns:xsi="http://ift.tt/ra1lAU">
    <targets async="true">
      <target xsi:type="File" name="f" fileName="path to file" />
    </targets>
    <rules>
      <logger name="*" minlevel="Debug" writeTo="c,f" />
    </rules>
  </nlog>
</configuration>

I'm trying to change the target's fileName value using Powershell.

I'm using this code:

[xml]$xml = Get-Content $file
$ns = New-Object System.Xml.XmlNamespaceManager($xml.NameTable)
$ns.AddNamespace("ns", $namespace) # $namespace is `http://ift.tt/JmPwGs`
$values = $xml.selectNodes($xpath, $ns)

My XPath expression is //ns:nlog/ns:targets/ns:target[@ns:name='f']/@ns:fileName

But $values always has 0 results.

Any idea ? What am I doing wrong ?

Thanks !