You are using an older version of SolrNet. This error is a known issue with older versions of SolrNet and Solr 4.X. Please grab an updated version from one of the following:
- NuGet - Pre-Release v0.4.0-beta2
- Solr Nightly Builds - (click the Artifacts link to get access to the downloadable zip)
- GitHub Source - pull the latest source down and compile locally or use a release.