help with add-to-list syntax

[BP25]

Hello! Could any of you be so kind as to help me understand this,
please? I have

(custom-set-variables
'(warning-suppress-types '((undo\ discard-info))))

in my .emacs. If I start emacs, do load-file .emacs, then evaluate
(add-to-list 'warning-suppress-types '(undo\ discard-info)) I get an
error saying the variable warning-suppress-types is void. However if I
start emacs, do load-file .emacs, then do C-h v warning-suppress-types
then I see the value being ((undo\ discard-info)) and if I evaluate
(add-to-list 'warning-suppress-types '(undo\ discard-info)) then
everything goes through without error. Why?!

Please, let me know! (and please, CC: my email address in your reply
because I'm not subscribed to this list)