Reporting Services :: How To Get A Blank Parameter To Work

Have a report that has this as the query:

SELECT SOURCE, TRANSDATE, LOCATION, DESCRIPTION, MOACTIVETIMESTAMP, MOINACTIVETIMESTAMP
FROM CTS_Missing_Data_Report_VW
WHERE LOCATION = @Location AND SOURCE = @Source AND (TRANSDATE BETWEEN @StartDate AND @EndDate)
ORDER BY Transdate desc, Location

So the user can enter a value for Location and Source and select the date range.BUT I also want the user to be able to put in nothing for the Location and Source so the query would return everything for that date range.

So if they did this the query would be

SELECT SOURCE, TRANSDATE, LOCATION, DESCRIPTION, MOACTIVETIMESTAMP, MOINACTIVETIMESTAMP
FROM CTS_Missing_Data_Report_VW
WHERE (TRANSDATE BETWEEN @StartDate AND @EndDate)
ORDER BY Transdate desc, Location

I have set the parameters @Location and @Source to "Allow blank value" in the datasets for the location and source I have :

SELECT NULL AS Source
UNION
SELECT DISTINCT RTRIM(LTRIM(SOURCE))AS Source
FROM CTS_OPS_SOURCE_LOCATION_TBL_VW
ORDER BY SOURCE

So a blank will show on the drop down and when I run the query for the Query Designer in the Dataset Properties the results does show  a blank record for the first record.BUT when I Run the report there are no blanks in the drop downs for the location or source. And there is no '<blank>' selection in the drop down either. And the drop down insist the user selects a value from both of the drop downs.

What am I doing wrong?