SQL & PL/SQL

Could any one help me in resolving the below error, I have this query in a procedure which is causing the group by error
v_z has assigned 0 in the begining

SELECT     DISTINCT 'I'  region_type, 
                b_no, 
                issuer_region_id, 
                issuer_country, 
                SUM(base_mkt_value) over(PARTITION BY b_no, issuer_country, issuer_region_id)mv, 
                v_z a109, 
                v_z a110 
FROM   B_RISK_POS 
WHERE  B_NO = 591
GROUP BY 
               'I' , 
                b_no, 
                ISSUER_REGION_ID, 
                ISSUER_COUNTRY;
                v_z;       

ORA-00979: not a GROUP BY expression
00979. 00000 - "not a GROUP BY expression"
*Cause:
*Action:
Error at Line: 5 Column: 24