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It's not defined in the spec what a valid local end-provided session ID is. Because section 2.1 says it defaults to null, which is distinguishable from undefined (i.e. not set), it follows that it cannot be null. This needs to be defined. (In my opinion we should change it so that sessionId to Command should be left undefined, that is optional. This would save the remote from having to do an explicit null check and would fix this bug because whatever value is passed in, even the null value, is what is used.)
See also bug 27766, and possibly bug 27765.