sub() with "+" - invalid regular expression

Hi,


can someone help me;
I don't understand why this code doesn't do what it's supposed to do;

x <- c("F+", "F+")
x <- sub("F+", "F", x)
x

[1] "F+" "F+"

(I want "F" "F")




when I try this;
x <- c("F+", "F+")
x <- sub("+", "", x)
x

I get an error message ("invalid regular expression")