Filters, use a conditional expression for the value using the IN operator

I want to filter a Tablix and the operator is "In".  if I use:

1,3

for the value, I see only the summary rows as expected.  if I use:

1,2,3

for the value, I see all rows as expected.

However, I want to use a condition based on a Boolean parameter being passed in, "ShowAll".  So I wrote my expression like this:

= iif((Parameters!ShowAll.Value=TRUE),"1,2,3", "1,3")

and the report wont run.  I get the error: "The size necessary to buffer the XML content exceeded the buffer quota.

How can I use a conditional expression for the value using the IN operator?

Thanks.