XPath exception in web.config edition

Hi,

I´m developing a feature to edit the web.config file. I have followed a few tutorials and finally I write this code

public class WebConfigRecieverEventReceiver : SPFeatureReceiver
    {

        private const string cWebConfigModificationOwner = "owner";

        SPWebConfigModification spWebConfigModification = new SPWebConfigModification()
        {
            // The owner of the web.config modification, useful for removing a
            // group of modifications
            Owner = cWebConfigModificationOwner,
            // Make sure that the name is a unique XPath selector for the element
            // we are adding. This name is used for removing the element
            Name = "bindings",
            //Name = "mySection[@name=\"Prueba\"]",
            // We are going to add a new XML node to web.config
            Type = SPWebConfigModification.SPWebConfigModificationType.EnsureChildNode,
            // The XPath to the location of the parent node in web.config
            Path = "/configuration/system.serviceModel",
            // Sequence is important if there are multiple equal nodes that
            // can't be identified with an XPath expression
            Sequence = 0,
            // The XML to insert as child node, make sure that used names match the Name selector
            Value = "<bindings/>"
        };

        public override void FeatureActivated(SPFeatureReceiverProperties properties)
        {
            SPWebService newService = SPWebService.ContentService;
            newService.WebConfigModifications.Add(spWebConfigModification);
            newService.Update();
            newService.ApplyWebConfigModifications();

        }

Debugging my code I have an exception when I try to aply the web config modifications. This is the exception:

An exception of type 'System.Xml.XPath.XPathException' occurred in System.Xml.dll but was not handled in user code
Additional information: '' is an invalid expression.

Anybody can help me? I don´t know where the fail is. I think it´s the web.config root but I´ve copied the same as other users have copied any the don´t have problems.

Thanks in advance.