The query notification subscription message is invalid

Hi!
I have some code that uses System.Web.Caching.Cache object to cache some data and invalidate them automatically using SqlDepenency. The command related with the SqlDependency is a simple select statement from a table. Although the code works smoothly most of the times, every now and then (1 in 5 cases) I get exception 'The query notification subscription message is invalid'. I see that this problem is only mentioned once in these forums without any answer.
Does anybody have any idea?

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Dimitris Papadimitriou, Software Development Professional