[Mono-list] ArgumentException in XmlTextWriter.WriteStartAttribute: Namespace URI must not be null when prefix is not an empty string

[Andrus]

In Mono 2.4.2.2 XmlTextWriter.WriteStartAttribute throws exception below is 
using open-source Yogesh ExcelXml class.
In .NET 3.5 same code works OK

How to fix ?

Andrus.

[System.ArgumentException]: Namespace URI must not be null when prefix is 
not an empty string.
  at System.Xml.XmlTextWriter.WriteStartAttribute (System.String prefix, 
System.String localName, System.String namespaceUri) [0x00272] in 
/root/src/mono-2.4.2.2/mcs/class/System.XML/System.Xml/XmlTextWriter2.cs:886
  at System.Xml.XmlWriter.WriteAttributeString (System.String prefix, 
System.String localName, System.String ns, System.String value) [0x00000] in 
/root/src/mono-2.4.2.2/mcs/class/System.XML/System.Xml/XmlWriter.cs:243
  at Yogesh.ExcelXml.XmlStyle.Export (System.Xml.XmlWriter writer) [0x00000]
  at Yogesh.ExcelXml.ExcelXmlWorkbook.Export (System.IO.Stream stream) 
[0x00000]
...