Invalid operator for data type. Operator equals add, type equals hierarchyid.

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SQL-Server

SQL-Server-2008

Hi,
i'm trying to transform my hierarchy using procedure as follows:

DECLARE @FilterId NVARCHAR(MAX) = 'CUSTOMER_IFRS'
DECLARE @unid as NVARCHAR(MAX) = '1410'
    DECLARE @id HIERARCHYID
        SELECT @id = HID FROM dbo.HIERARCHIES WHERE  FilterId =@FilterId  and id = @unid;


DECLARE @SQL NVARCHAR(MAX) = 'SELECT OFSA_ID as '+@FilterId+', sName AS LEAF_DESCRIPTION'
DECLARE @ProductMaxLevel as NVARCHAR(MAX)
SELECT @ProductMaxLevel = MAX(HID_Level)  FROM HIERARCHIES WHERE FilterId = @FilterId


DECLARE @usableLevels NVARCHAR(MAX)
DECLARE LevelHierarchy CURSOR FOR
    SELECT DISTINCT HID_Level FROM HIERARCHIES WITH (NOLOCK) WHERE FilterId = 'CUSTOMER_IFRS'  order by 1 asc;
            OPEN LevelHierarchy;
                FETCH NEXT FROM LevelHierarchy INTO @usableLevels;
                    WHILE @usableLevels <= @ProductMaxLevel-1
                        BEGIN


        SET @SQL = @SQL +'
        ,(SELECT a.Ofsa_id FROM  dbo.HIERARCHIES a  where  a.FilterId ='''+@FilterId+''' and  '+@id+'.IsDescendantOf(a.HID) = 1 and a.Tree_Level = '+@usableLevels+') AS LEVEL_0'+@usableLevels+'_ID,
         (SELECT a.sName FROM  dbo.HIERARCHIES a  where  a.FilterId ='''+@FilterId+''' and  '+@id+'.IsDescendantOf(a.HID) = 1 and a.Tree_Level = '+@usableLevels+') AS LEVEL_0'+@usableLevels+'_DESC'


                                --PRINT @SQL
                                PRINT @usableLevels




                                FETCH NEXT FROM LevelHierarchy INTO @usableLevels;
                    END;
                    CLOSE LevelHierarchy;
                    DEALLOCATE LevelHierarchy;




        SET @SQL =@SQL +' FROM HIERARCHIES Where FilterId ='''+@FilterId+''' and id = '+@unid+''
        --EXECUTE sp_executesql @SQL
        SELECT @SQL

Unfortunatelly, i'm getting error

Msg 403, Level 16, State 1, Line 22
Invalid operator for data type. Operator equals add, type equals hierarchyid.

I think the error is related to part where I'm trying to pass @id parameter into SQL Statement ('+@id+'.IsDescendantOf(a.HID) = 1)

If anyone can help me in order to to solve the problem.
Any advice/help/code corecction would be very much appreciated.
Thanks
A