[R] Checking if a logical variable exists

[Steven Yen]

My obj does not always come with a logical variable defined. So I do

my.foo <- function(obj,df,digits=5){
if (!is.na("obj$spec$Fisher")) Fisher<-obj$spec$Fisher
...
}

This works when "Fisher" is defined in/passed from obj. When it is 
not, I get error:

Error in (!is.na("obj$spec$Fisher")) & Fisher :
   operations are possible only for numeric, logical or complex types

I tried exist(Fisher), missing(Fisher)... to no vail. Any idea? Thanks.