Multi page zend forms with file type element

Question

I have made several multi page forms based on the example from zend official site: zend form advanced

As long as I use text, radio or select elements everything is going OK. When I am trying to use file type element I get error and message:

Invalid argument passed to prepareSubForm()

I think, it is worth to write that when I have three sub forms for example 'first', 'second', 'third' attached to main form and file type element is included in third (last one) sub form then this problem not exist (maybe is better to write that this problem is not shown).

Why putting file type element in second sub form and after submitting last form I get errors?