set histchars not working in zsh

Consider the following commands for overriding the history expansion characters (which I got from *Learning Shell Scripting with ZSH* book):

% set histchars='@^#'
% ls *.txt
file1.txt  file2.txt
% @@

Result:

zsh: command not found: @@

However, this works (note that set is omitted):

% histchars='@^#' 
% ls *.txt       
file1.txt  file2.txt
% @@

Result:

ls *.txt
file1.txt  file2.txt

I tried both with my .zshrc and with a vanilla shell (zsh --no-rcs), the result is the same. Question: is the set ... syntax wrong in the book, or am I missing something?