false alarm on defcustom initialized to a value

[Fuco1]

(defcustom pomidor-confirm-end-break t
  "If t ask for confirmation before ending a break and starting new a pomodoro."
  :type 'boolean :group 'pomidor)

(or (not pomidor-confirm-end-break)
                    (yes-or-no-p "Stop break and start new pomidor?"))

Elsa thinks that (not pomidor-confirm-end-break) always evaluates to nil but this is not the case since the variable is actually a boolean despite the default assignment.