Error in : object of type ‘closure’ is not subsettable

I was finally able to work out the code for my scraping. It seemed to be working fine and then all of a sudden when I ran it again, I got the following error message:

Error in url[i] = paste("http://en.wikipedia.org/wiki/", gsub(" ", "_",  : 
  object of type 'closure' is not subsettable

I am not sure why as I changed nothing in my code.

Please advise.

library(XML)
library(plyr)

names <- c("George Clooney", "Kevin Costner", "George Bush", "Amar Shanghavi")

for(i in 1:length(names)) {
    url[i] = paste('http://en.wikipedia.org/wiki/', gsub(" ","_", names[i]) , sep="")

    # some parsing code
}