On .NET, the code for wrapping a DOM (System.Xml.XmlDocument) in a Saxon NodeInfo interface wrapper cannot handle nodes of type Whitespace or SignificantWhitespace.
A patch (untested but low-risk) will be placed in Subversion. Affects module net.sf.saxon.dotnet.NodeWrapper
Workaround: copy the contents of the DOM Document to a newly constructed Saxon tree. In some cases this may give better performance anyway.