Functions returns empty set

Functions returns empty set

I have successfully created a function in SQL 2008:
ALTER FUNCTION [dbo].func_RAW_data_xref
(
@ColName as Nvarchar(255)
)
RETURNS @VVSrcCDs TABLE
(
vv_SRC_CD NVARCHAR(255) NULL,
vv_CD NVARCHAR(255) NULL,
vv_SRC_CD_DESC NVARCHAR(255) NULL,
vv_DSC NVARCHAR(255) NULL
)
AS
BEGIN
DECLARE
@vv_SRC_CD NVARCHAR(255),
@vv_CD NVARCHAR(255),
@vv_SRC_CD_DESC NVARCHAR(255),
@vv_DSC NVARCHAR(255);
SELECT
@vv_SRC_CD = A.vv_SRC_CD,
@vv_CD = A.vv_CD,
@vv_SRC_CD_DESC = A.vv_SRC_CD_DESC,
@vv_DSC = A.vv_DSC
FROM DBO.values A
JOIN VVLookup B
ON A.vv_SRC_CD = b.vv_SRC_CD
WHERE B.[Client Column Name] = @ColName
RETURN;
END
The problem is when I call the function:
select * from dbo.func_RAW_data_xref('_CD');
I get no results. What I do get are the columns referenced in the function
but no data. If I copy the select statement out of the function and run it
with a valid parameter for @ColName, I do get results.