Function Error

When I call this function from a select statement 
select f_rec_cnt(table_name) from <AnyTable> -- it is always returning null and no rowcount as the table contains data init
NULL
The AnyTable contains table_name column with value as actual table name in the database, I want the
function to return the number of records in that table.


ALTER FUNCTION  Collect.f_rec_cnt ( @TableName VARCHAR(100))
RETURNS INT
AS
BEGIN
DECLARE @rowCnt INT 
DECLARE @TableNm varchar(100)
SET @TableNm=''@TableName+''
SELECT @rowCnt = ROWS FROM sys.partitions p
                inner join sys.tables t
        ON p.object_id = t.object_id
        WHERE t.name = @TableNm
        and index_id =1
        RETURN @rowCnt
END

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Neil