Using a multi parameter with OR to determine visibility

Hi,

I have a parameter that controls the visibility of may tables.  I am having trouble getting the expression correct as it needs to be like this:

If parameter contains 'this value' OR 'this value' then true,false.

I know I need to use Instr(Join somehow.  This is what I have so far:

=IIf(InStr(Join(Parameters!SortBy.Value, ","), "c") > 0 or
Parameters!SortBy.Value = "Subject",false, true)

What have I got wrong!?

Thanks is advance!