Oops, I had to change my Parameter in the stored proc to match that of the original parameter's type (string = varchar).
Primary report parameter = CustID (string).
Stored Proc now has CustID (varchar).
Verified my subreport to pick up the change.
On main report, right click subreport and choose Change Subreport Links.
In available fields, select the formula field you made that reflects the value of your primary report parameter (CustID).
In the 'Subreport Parameter field to use' select the parameter from your subreport. (?@CustID, NOT ?Pm-CustID!).
That should do it. At least it did for me. Hope that works!