title: Cannot use a prefix with an empty namespace
body: <p>I am creating a XML document in c#.</p>
  
  <p>Its all coming together now but for some reason i keep getting tis error.</p>
  
  <p>Here is my code:</p>
  
  <pre><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></pre>
  
  <p>The error keeps appearing on the "a" prefix on the start of the method.
  Any help will really be appreaciated.</p>