If you are using a keyboard layout that doesn't have the backtick, use \@ instead of @. That avoids the character is thought to be a @-reference, and allows it to be used.
I am using the Italian keyboard layout, and the keyboard doesn't have the backtick. (I have to use the Italian 142 layout, and switch to it when I need to print a back-tick.)
Doing this shows up the backslash, but if you don't have a keyboard with the backtick, this at least avoids the warning message.
If you are using a keyboard that allows you to enter a backtick without changing keyboard layout, since you are referring to a variable name, you can simply wrap the variable name with backticks (e.g.
@variablename) which works on comments too.
In my case, I switch to the Italian 142 layout and press alt-gr+ù just for the time to write the backticks; then I switch back to the usual Italian layout, which allows me to enter #, [, or ].