Extension functions cannot return null values.

[XslTransformException: Extension functions cannot return null values.]
   System.Xml.Xsl.Runtime.XmlQueryRuntime.ChangeTypeXsltResult(XmlQueryType xmlType, Object value) +1002628
   System.Xml.Xsl.Runtime.XmlQueryContext.InvokeXsltLateBoundFunction(String name, String namespaceUri, IList`1[] args) +830