Modal form from modal form

[C# Programming]

I have a windows forms application which does invoicing among other things.

When a new invoice line is created the user can request a modal form with article numbers. On this article select form I want to create a "new" button which loads another modal form on which the new article is defined.

C# says it can't launch a modal form from a modal form.

Does anyone have a solution ?
I'm flexible in architecture of the application so a different approach is fine.

Thanks in advance,

Rob