[Jprogramming] prob with redefining hook

['Pascal Jasmin' via Programming]

hook =: 2 : ('([: u v)';':';'(u v)')

no problems with dyad:

   4  + (hook +:)(hook +:) 2 3 
12 16 

but why does this not work?
     +: (hook +:)(hook +:) 2 3 
|domain error