When using Enterprise Library Blocks you may end up with this error. I used to have a blog entry on all of the specifics but I've lost that entry when my DB crashed several years ago. I am dating this blog entry for that time period and who knows, I may yet try to recover my old DB.
Try the following to see if the issue can be fixed:
HTHCross posted from my blog at http://schema.sol3.net/kbarrows