Lookup Error - SQL Server Database Error: The multi-part identifier "AG.AGREE_NUM" could not be bound.

Data from  Agreement and Agreement_Items  is  exported to  two  tables daily. On occasions number of Agreement_Items  per an agreement  in the imported  table  is not matching the  count in the source table. I  need to display/compare  the counts .  unfortunately my query gives me  error: Lookup Error - SQL Server Database Error: The multi-part identifier "AG.AGREE_NUM" could not be bound.
       
SELECT SIEB.AGREE_NUM, SIEB.SIEBEL_COUNT, SAP.SAP_COUNT
FROM
(    
SELECT  AG.AGREE_NUM , COUNT (AGI.DOC_AGREE_ID) AS SIEBEL_COUNT
FROM Siebel.dbo.S_DOC_AGREE AG , Siebel.dbo.S_AGREE_ITEM AGI
WHERE AG.ROW_ID  = AGI.DOC_AGREE_ID AND AG.AGREE_CD = 'Service Contract'
GROUP BY AG.AGREE_NUM
) SIEB
INNER JOIN
(
SELECT sp.VBELN, COUNT(cl.VBELN) AS SAP_COUNT
FROM SAP.dbo.VBAK sp , SAP.dbo.VBAP cl
WHERE sp.VBELN = cl.VBELN AND  sp.AUART = 'ZWV1'  
GROUP BY sp.VBELN
) SAP
ON AG.AGREE_NUM = sp.VBELN
ORDER BY SIEB.AGREE_NUM