Outlook interop getting "0x80020003 (DISP_E_MEMBERNOTFOUND)" error when processing appointment exceptions

I have a piece of code to process appointment changes in Outlook. If the appointment is a recurring one then the code retrieves the RecurrencePattern and go through the items in the Exceptions list (this "exception" refers to "appointment exception" meaning one of the appointment in a recurring series is changed/edited).

The code needs to store some extra information in the appointment's UserProperties. I found that the exception appointments all have the same user property value as the master recurring appointment, then I found that the appointment exception object also has a property called ItemProperties. According to MSDN, this is pretty much the user property collection for each appointment exception.

However, whenever I try to access this ItemProperties property, I get a COM exception saying: Member not found. (Exception from HRESULT: 0x80020003 (DISP_E_MEMBERNOTFOUND)).

Does anyone have any idea how I can store and read custom information against each appointment exceptions?

Store the data on the master appointment. You can have a binary property that stores the data in an <original date> vs <your data> dictionary.