SQL & PL/SQL

Hi all,

I have written the following query . But I am geting the Column ambiguously error while compiling. I am not able to find the eror. Can any one please correct this.

SELECT
DISTINCT SDN,CODELANG
FROM DSS.ZONE_HZ,SCS.CONTR_SERVICES
WHERE CO_ID IN ( SELECT CO_ID
                 FROM DSS.ZONE_HZ
                 MINUS
                 SELECT CO_ID
                 FROM DSS.FORT_HZ
               )
AND ZONE_HZ.CO_ID = CONTR_SERVICES.CO_ID
AND SUBSTR (NVL (contr_services.cs_stat_chng, 'a'), -1, 1) != 'd'
AND SCODE NOT IN (2529,2530);

Thank you