Parsing Amzon API *.xml response with .NET

I'm trying to parse the xml-response from the amazon api.

This is a part of the received xml-file:

<BrowseNodeLookupResponse xmlns="http://ift.tt/JgRJni">
  <OperationRequest>
    <RequestId>31317fca-ad3d-4ff0-a64f-693c0e44959b</RequestId>
    <Arguments>
      <Argument Name="Operation" Value="BrowseNodeLookup" />
      <Argument Name="Service" Value="AWSECommerceService" />
      <Argument Name="Version" Value="2011-08-01" />
      <Argument Name="BrowseNodeId" Value="186606" />
      <Argument Name="Timestamp" Value="2015-01-04T11:50:06Z" />
      <Argument Name="ResponseGroup" Value="BrowseNodeInfo" />
    </Arguments>
    <RequestProcessingTime>0.002221</RequestProcessingTime>
  </OperationRequest>
  <BrowseNodes>

I want to read the Argument Timestamp. This is my code, but it only work if I remove the xmlns attribute in the xml-file.

    Dim nodeTimestamp As XmlNode = doc.SelectSingleNode("/BrowseNodeLookupResponse/OperationRequest/Arguments/Argument[@Name='Timestamp']")

    Dim text As String = nodeTimestamp.Attributes.ItemOf("Value").InnerText