Content is invalid: 'null' whenever a url is entered - Calais.net

I am using the Calais.net library to pass content to the OpenCalais webservice, returning a JSON document to process.
I can pass in nearly every text file I have but when I try to pass in a URL I get the "Content is invalid: 'null'" error virtually every time, (maybe 1 time in 10 works).
The following is an example of the URL passed in, the xmlParams and the error that is returned.
Can anyone let me know why this is happening please.
Regards
D James
http://missoulian.com/news/state-and-regional/article_68c3660a-e32f-11e0-a48d-001cc4c002e0.html xmlns:c=\"http://s.opencalais.com/1/pred/\">\r\n  <c:processingDirectives c:contentType=\"text/htmlraw\" c:outputFormat=\"application/json\" c:reltagBaseURL=\"\" c:enableMetadataType=\"\" c:calculateRelevanceScore=\"false\" c:discardMetadata=\"\" />\r\n  <c:userDirectives c:allowDistribution=\"true\" c:allowSearch=\"true\" c:externalID=\"\" c:submitter=\"\" />\r\n  <c:externalMetadata />\r\n</c:params>"
"<c:params
"<Error Method=\"ProcessText\" calaisRequestID=\"815291f4-013e-832e-1328-594e42346860\" CreationDate=\"2011-09-20 01:45:20\" CalaisVersion=\"R4.3_7.1.1103.5\"><Exception>content is invalid: 'null' .</Exception></Error>"