change parameter to 'ALL' when is NULL

I want to change the nulls to 'All' instead of showing NULL in the parameter.  I have 'Allow NULL values option selected'

This is my dataset for a parameter I am using for the parameters
--this data set shows null and works fine when rpt is exec
select distinct  Terr from dimcustomer
order by Terr
--this does not work, it thinks 'All' is not null
select distinct  isnull(Terr,'All') as Terr from dimcustomer
order by Terr

Any suggestions