[Jprogramming] Why does 1 : ' x u/\ y' fail?

[Piet Google]

Since

    2 +/\ i.5
1 3 5 7

Further

    ad=.1 : 'u/\'
    2 + ad i.5
1 3 5 7

However 

    adv=.1 : 'x u/\ y'
    2 + adv i.5
|domain error

Why?  I suspect it is something to do with bracketing and/or ‘x’ being passed to u.