Oracle 10G: ORA-06575: function in invalid state

I created a funcion like this

CREATE OR REPLACE FUNCTION tax 
(p_sal IN  NUMBER(4)) 
RETURN NUMBER 
AS
v_tax NUMBER(4);
BEGIN 
v_tax:= CASE        
WHEN p_sal> 4000 THEN
p_sal*0.33       
WHEN p_sal >2500 THEN 
p_sal*0.25      
WHEN p_sal >1500 THEN 
p_sal*0.20     
ELSE 0         
END; 
RETURN v_tax;
END;
/

when i used this tax function in insert stmt like

INSERT INTO employees(eno, ename, job, join_date, sal, comm)
VALUES (7784,'allen','salesman',sysdate, 5000, tax(5000));

it shows the error like

ERROR: ORA-O6575: package or function tax is in invalid state.

can anyone suggest me how to make this function is in valid state? thanks in advance.