If, one day, you're creating a linked report, and you specify a default value for one (or more) of the parameters, you might receive an error stating 'The parameter [name] is missing a value' when you test that the report is working.
No matter what you try, nothing works until you decide to check the XML in the Parameter column in the table ReportServer.dbo.Catalog. You'll need to remove this particular string:
This key/value causes the report to basically delete the default value you've specified for the parameter, hence the error message. Just copy the current XML string from that field, remove that string, and update it with the XML you've just modified.