brain cramp or bug: inner join returns rows w/null

i *thought* inner join should not return nulls, but not only that, but i get way more rows than i'm expecting.

assume:

order table:
  order_number

order_line table:
  order_number
  order_line_number
  value

assume 1,000 order rows, 20,000 order_line rows all have an

                         order_number and value.

the following:

select value from order_line, order
  where order_line.order_number = order.order_number

returns rows with value = null

??