Error while generating XML document

I am using the following C# code to create a XML file:

private void CreateXML()
{
    _logger.Log("Creating XML....");

    XmlWriterSettings xmlWriterSettings = new XmlWriterSettings
    {
        Indent = true,
        OmitXmlDeclaration = false,
        Encoding = Encoding.UTF8,
        CheckCharacters = true
    };

    XmlSerializer serializer = new XmlSerializer(typeof(List<Page>));

    using (XmlWriter writer = XmlWriter.Create(_outputXMLFilePath, xmlWriterSettings))
    {
        serializer.Serialize(writer, this._pages);
    }

    _logger.Log("XML Created");
}

On line serializer.Serialize(writer, this._pages); I am getting the following exception:

An unhandled exception of type 'System.InvalidOperationException' occurred in System.Xml.dll

Additional information: There was an error generating the XML document.

Can anyone please tell me what is the issue here?