Cannot use a prefix with an empty namespace

[BartBon]

I am creating a XML document in c#.Its all coming together now but for some reason i keep getting tis error.Here is my code:\[code\] FileStream Slide2XmlStream = new FileStream(slide2Xml, FileMode.Create, FileAccess.Write); XmlWriter slide2XmlWriter = null; try { XmlWriterSettings contentTypesSettings = new XmlWriterSettings(); contentTypesSettings.Indent = true; slide2XmlWriter = XmlWriter.Create(Slide2XmlStream, contentTypesSettings); slide2XmlWriter.WriteStartDocument(true); slide2XmlWriter.WriteStartElement( "p", "sld","http://schemas.openxmlformats.org/presentationml/2006/main" ); slide2XmlWriter.WriteAttributeString("r", "http://schemas.openxmlformats.org/officeDocument/2006/relationships"); slide2XmlWriter.WriteAttributeString( "a","http://schemas.openxmlformats.org/drawingml/2006/main"); slide2XmlWriter.WriteStartElement( "p", "cSld", null); slide2XmlWriter.WriteStartElement( "p", "spTree", null ); slide2XmlWriter.WriteStartElement( "p", "nvGrpSpPr", null ); slide2XmlWriter.WriteStartElement( "p", "cNvpr", null ); slide2XmlWriter.WriteAttributeString("name", ""); slide2XmlWriter.WriteAttributeString("id", "1"); slide2XmlWriter.WriteEndElement(); slide2XmlWriter.WriteStartElement( "p", "cNvGrpSpPr", null ); slide2XmlWriter.WriteEndElement(); slide2XmlWriter.WriteStartElement( "p", "nvPr", null ); slide2XmlWriter.WriteEndElement(); slide2XmlWriter.WriteEndElement(); slide2XmlWriter.WriteStartElement( "p", "grpSpPr", null ); slide2XmlWriter.WriteStartElement( "a", "xfrm", null ); slide2XmlWriter.WriteStartElement( "a", "off", null ); slide2XmlWriter.WriteAttributeString("y", "0"); slide2XmlWriter.WriteAttributeString("x", "0"); slide2XmlWriter.WriteEndElement();\[/code\]The error keeps appearing on the "a" prefix on the start of the method.Any help will really be appreaciated.