Error when trying to Extend PerformanceDetailsVO

When I am trying to modify the the column in PerformanceDetailsVO
 Original :- decode(ppr.performance_rating, null, null,
          ppr.performance_rating||' - '||hr_general.decode_lookup('PERFORMANCE_RATING',ppr.performance_rating)) last_rating,

need to modify :- hr_general.decode_lookup('PERFORMANCE_RATING',ppr.performance_rating) last_rating,

I am getting error like "each row in the query result columns must be mapped to a unique query attribute in the Mapped Entity Column"

I even tried by checking the box "Updatable Access through Entity Objects" still getting the same error.