Need subquery

I need a subquery require to copy a content into another table of same field names. The table1 having 100 records and table2 having 50 records

 Query is :

  update stock1 t1 set t1.amt = (select t2.amt from stock2 t2 where t1.pr_no = t2.pr_no)

  The amt column is not null constraint, If running this query it shows Null value error

What is the reason, explain me. Received on Mon Jun 28 2004 - 08:15:52 CDT