为什么每次尝试运行时都会收到“策略谓词有错误”？

Question

我写了如下函数：

create or replace function function1 (schema_obj varchar2, tbl_obj varchar2)
return varchar2 is
r_es varchar2(1000);
BEGIN
r_es := 'opo.p_id= (select p_id from mo.pato where upper(pname)=
(select  sys_context(''userenv'',''session_user'') from dual))';
RETURN r_es;
end function1;    

并将其添加到vpd策略中。

但是当我运行任何命令时，出现以下错误：

ORA-28113: policy predicate has error
28113. 00000 -  "policy predicate has error"
*Cause:    Policy function generates invalid predicate.
*Action:   Review the trace file for detailed error information.
Error at Line: 1 Column: 18