Nim 2: What’s special about Task?

[hamidrb80]

I've looked at std/tasks page in the stdlib, and cannot understand why's first 
block does not work:
    
    
    import std/tasks
    
    type
        Runnable = ref object
            data: int
    
    proc hello(a: Runnable) =
        a.data += 2
    
    block:
        let x = Runnable(data: 12)
        let b = toTask hello(x) # error ----> expression cannot be isolated: x
        b.invoke()
    
    block:
        let c = toTask(hello(Runnable(data: 12)))
        c.invoke()
    
    
    
    Run

simply because in the first block I initialized `Runnable` outside of arguments 
list?