help with tclVar

Hi, I can' t load a variable tcltk declared with tclVar, why is this?, the
exmple above explain me ,Thanks Ruben
a<-tclVar(init="")

f<-function(){

+ a<<-"pipo"
+ }

f()
a

[1] "pipo"

tclvalue(a)

Error in structure(.External("dotTcl", ..., PACKAGE = "tcltk"), class =
"tclObj") :
        [tcl] can't read "pipo": no such variable.