Button icon in tree

How can I add an icon to button in tree? It works for the form, but in tree it is not showing and there’s an error during module upgrade:

trytond.ir.ui.view.XMLError: Invalid XML for view "mymodule.mymodel (Tree)". - 
ERROR:RELAXNGV:RELAXNG_ERR_INVALIDATTR: Invalid attribute icon for element button

It is not supported by the client so it is not valid.