How to include the null columns only?

Dear all, i have the following query:

select sf.student_no, sf.fee_month, sfp.amount
from stu_fee sf, stu_fee_payment sfp
where sf.student_no = sfp.student_no(+)
and sf.fee_month = 'MAY-2010'
and TRUNC(sfp.pay_date(+)) = TO_DATE('MAY-2010','MON-YYYY')

it gives the following result:
   Student_no   fee_month            amount
    1                MAY-2010                100
    2                MAY-2010                100
    93946         MAY-2010       
    93939         MAY-2010       
    30              MAY-2010       
    93953         MAY-2010       
    93941         MAY-2010       

now I want to display only the records with null amount;

Regards:

Muhammad Nadeem