Sort list-items in dropdown box

In the editform of a list I have added a lookup control linked to another list (list of months).

The list 'Months' has 2 columns 'Title' (name of month) and 'MonthNo' (number of month).

When I open the dropdown list the months are sorted on alphabet (so April, August etc) instead of the correct order.

How can I stop the dropdown box from sorting the list (the items in the list are added in the correct order), without having to put a order-number in front of the monthname? (like 01 January, 02 February etc).

In the 'sort and group' actions, I've already ascend开发者_Go百科ing sorted on MonthNo, but the dropdown list seems to ignore that.

Thanks.