function异常

function错误
昨天下班的时候，测试爆出一个错误：
问题出在下面这个functions的第九行：

CALCNPV functions:CREATE OR REPLACE FUNCTION "GAOFEI"."CALCNPV"(monthFv in number, recoveryExp in number, monthTime in number, irr in number)      RETURN number IS netRec number;BEGIN  if monthFv is null or recoveryExp is null or monthTime is null or irr is null  or monthTime > 1395 then    return null;  end if;  netRec := ISNULL_NUMBER(monthFv, 0) * (1 - ISNULL_NUMBER(recoveryExp, 0));  return netRec / power   (1 + ISNULL_NUMBER(irr, 0), ISNULL_NUMBER(monthTime, 0) / 12);END;