Setting Collection Element Value Using Expression Fails

I'm trying to set the value of the SelectedDatabases collection property of an Update Statistics Task using an expression for the task, basing it on a user variable. Ultimately, I'll set that variable's value from a configuration file at run time and thereby specify the database whose statistics I want to update.

When I set this up in the designer, I get an error at the time I try to save the package:

TITLE: Microsoft Visual Studio
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Nonfatal errors occurred while saving the package:
Error at Update Statistics Task: The result of the expression "@[User::SelectedDatabase]" on property "SelectedDatabases" cannot be written to the property. The expression was evaluated, but cannot be set on the property.

Error at Update Statistics Task: The result of the expression "@[User::SelectedDatabase]" on property "SelectedDatabases" cannot be written to the property. The expression was evaluated, but cannot be set on the property.




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BUTTONS:

OK
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I used the expression editor's expression builder to select the user variable, so I assume the syntax is correct. However, I'm suspecting that there's some additional syntax required to store that value in the collection's array. I cannot find any references to any special formats and am wondering if anyone know's how to do this correctly. On the other hand, maybe it is telling me setting this property from an expression is not even allowed. If it's not allowed, why is it in the list of properties to set? In any case, none of this is clear.

Any ideas or suggestions are appreciated. Thanks,

Joe