[Vala] printf question

[wolfgang mauer kabelmail de]

Is this a Bug?
I tried this

cellRendererText.text = ("%"+int64.FORMAT).printf((int64)obj); <<---- int64.FORMAT = "li"

this works,

but when i try this, witch is the same
cellRendererText.text = ("%li").printf((int64)obj);

i get thiss error 

application.vala:192.45-192.54: error: Argument 1: Cannot convert from `int64' to `long'

???