fxRuby: changing icon in TreeItem crashes

I would like to change the icon used by TreeItem in TreeList.
I’m using

Convenience function to load & construct an icon

def makeIcon(filename)
begin
filename = File.join(“icons”, filename)
icon = nil
File.open(filename, “rb”) { |f|
icon = FXPNGIcon.new(getApp(), f.read)
}
icon
rescue
raise RuntimeError, “Couldn’t load icon: #{filename}”
end
end

doc = makeIcon(“minidoc.png”)
@tree.currentItem.text = “new icon coming …”
@tree.currentItem.closedIcon = doc # crash!

I had no problem adding icons when the TreeItem was constructed.

I’m using fox14.

Christer