[R] Saving with tkgetSaveFile

[solares@unsl.edu.ar]

HI, i'm trying to save a data frame with the next script:
x<-c(1,2,3)#suposse here the data frame
a<-tkgetSaveFile()
a<-tkgetSaveFile()
save(x,file=as.character(a))

but i obtain the next warning message:
Warning messages: 
1: the condition has length > 1 and only the first element will be used in: 
if (file == "") stop("`file' must be non-empty string") 
2: only first element of `description' argument used

and nothing file is saved, ¿What is the error? Thanks Ruben