SSRS how to allow both null and multiple values

Hi

I have an SSRS (SQL 2008 R2) report that has a parameter that pulls values from a query with the workaround of UNION select NULL  to be able to select NULL.  I would like to allow the parameter to be able to select one value, a NULL value, or multiple values.  However, SSRS does not allow me to select both allow null and multiple values.

There are a lot of varied solutions returned by Google, but I would like to know what our community has to say on the matter.

Thanks in advance,

James